You can only start a journey from where you're currently standing, not where you wish to be, and you have to start moving in the right direction from the place your feet are right now. The concept applies precisely fitness training, not in the spatial sense of location, but rather where you are at this moment in your training "career" in terms of time and the amount of adaptation you have already undergone.
The purpose of training is to cause (various) adaptation(s), and the amount of adaptation(s) that has already occurred determines the trainee's (your) training stage.
Your current training stage is the first metric to determine when defining your current location along the timeline of your training career. We break Training "stages" into three bird's eye categories:
Beginner
Intermediate
Advanced
The difference in the three stages lies in the amount of Loading (hence time) required to cause enough adaptation in a trainee to produce a noticeable gain in performance.
The Beginner Stage
A beginner is very far from reaching their genetic potential because no (or few) adaptation(s) has occurred. For this reason, only a minimal load is needed to engage the overload-recovery-adaptation process. A single session with 3-5 sets of two major exercises (Military Press or Bench Press and Olympic Squat or Powerlifting Squat) and 24-72 hours of recovery is sufficient to cause a noticeable performance gain in a beginner. Mark Rippetoe's Starting Strength is an excellent example of an optimal beginner program; basically, the exercises (Bench Press and Powerlifting Squat), sets (3-5), and reps per set (5) are kept stable session to session. The only change is in the weight, by adding 5 pounds (or 2-3 KG) on Bench Press and 10 Pounds (or 4-5 KG) to the squats each session. This slight increase in weight is enough to drive further adaptation(s) in a beginner every session; this is the fastest way to improve and the fastest stage of your training career in terms of the rate of gain) in performance.
It is essential to note the high efficiency of changing only the weight while keeping the exercises, sets, and reps stable. In any scientific process (which training is), it is most efficient to change as few variables as possible at any given point in time; this allows the scientist (trainer/coach) to have the best possible control over the process (the trainee's/your improvement). If the coach changes several variables once, it becomes tough to determine which change caused an improvement or regression, making it difficult to regulate the process. Remember, your purpose in the gym is to train, not seek novelty through random exercise.
The Intermediate Stage
At the intermediate stage, a trainee requires a more significant load/amount of time to cause improvement because they/you have now undergone a larger amount of adaptation(s) and are a bit closer to your genetic potential. Often the exercises are still kept stable with maybe one or two more accessory exercises added in (pull-ups and sit-ups are most common), but the weight-rep-set scheme is changed and spread across three sessions over a week. Usually, one day has heavy weights with low reps (3 or less) and 3-5 sets, while another day has moderate weights with moderate reps (5-6) and 3-5 sets; the intermediate stage sees the addition of the de-load or off-load concept. One day of the week (at the middle or end) utilizes light weights with moderate reps as active recovery; lifting with light weights and moderate reps maintains neural integration while allowing structural (musculoskeletal: muscles and bones) and vegetative (cellular energy) to fully recover between Loading or "stress" sessions. The intermediate stage also sees the emergence of the "cycle" concept in your training journey. The weekly cycle described here can be known as a "Microcycle."
Microcycle: A microcycle is a system of workouts that achieve a particular training objective.
The Advanced Stage
Once you reach the advanced stage, you will need a much larger load to cause improvement. Training cycles will now consist of multiple weeks (microcycles), and this group of weeks is called a mesocycle.
Mesocycle: A mesocycle is a system of microcycles that achieve a particular training objective. It usually contains several microcycles that develop and stabilize (and sometimes test) specific abilities in a given period according to a trainee's time-sensitive goals. A mesocycle always ends with at least one restorative (de-load) microcycle.
At this stage, the trainee/you have undergone much adaptation, and the goal of each mesocycle will be to inch closer and closer to full genetic performance potential. The exercises will remain mostly stable from microcycle to microcycle within a single mesocycle. Still, there will likely be several more exercises than at the intermediate stage, and the weights (intensity), reps, and sets will vary in complex ways across the mesocycle.
If your intermediate program consisted of Bench Press, Powerlifting Squat, Pull-Up, and Sit-Up, it might now include all those plus Power Snatch, Power Clean, Deadlift, and Military Press. The weights and reps-sets will vary on all of these across the mesocycles. Clearly, the complexity of the training process increases significantly at this stage.
At the advanced stage, you start to specialize your program (especially the intensities and sets-reps and the exercises to an extent) according to your training purpose. You can organize the training across time to ensure performance "peaks" according to specific goals. At this point, the concept of Periods may come into play; we call this type of training organization Periodization.
We organize Periods into a macrocycle that usually includes an athletic in-season and off-season, which we can further divide into more specific periods according to the athletic schedule's complexity. A macrocycle is generally more than four months and may often be a whole year.
Period: Periods are the basic building block of a macrocycle. A period is a system of mesocycles that are structured and sequenced to realize a particular timely training objective(s).
There is no point worrying about periodization at the beginner or intermediate stages. There has not been enough adaptation to develop abilities enough for a peak to even be possible at these stages, and you simply need to get stronger until you have reached the advanced stage.
You will know when you need to progress from one stage to the next when you can no longer improve from cycle to cycle. At the beginner stage, this means you can no longer increase the weight each session on all the lifts. At the intermediate stage, it means you can no longer increase the weights from week to week (microcycle to microcycle). It can be a bit more complex than this, but this is the basic concept.
It is crucial not to move on from one stage to the next too soon. Remember, you make the fastest improvement (day to day) at the beginner stage, then the intermediate (week-to-week), and finally, improvements take a month or longer at the advanced stage.
Advanced training has more variation and novelty, but don't let this "more fun" element entice you to slow your progress before necessary.
Armed with this information, you can now figure out where you currently stand. Once you know, you'll be able to start "moving" forward on your journey to personal fitness success.