In the simplest sense, "periodization" just means a training plan that builds on itself to achieve specific desired outcomes. Just like a skyscraper or house isn't just randomly flung together, a lifting program needs "direction". That's what periodization supplies. A building plan is layered step by step on top of itself: foundation dug, foundation laid, exterior framework, individual floor support, then interior....etc right down to the finish.
Periodization describes various ways of doing that with sports or strength goals. It is a way to try to relate the different variables so they build on each other rather than being randomly thrown together.
General elements in a training plan are:
hypertrophy/size (can be broken down into both overall and specific muscle group goals or needs)
Each of these has different training needs depending on your goals and sport. Obviously, you can't train for a marathon the same way you would train for a sprint right? Well neither can you train for a sprint the same way you would train for powerlifting or bodybuilding. Or an mma fight to a sprint. They're all different--"endurance" for powerlifting means something ENTIRELY different than for half marathon running, or even fighting or sprinting.
In the powerlifting example, a powerlifter's "endurance" would mean something along the lines of: 1 - their ability to continue straining for seconds on end in a lift (complete a grinding squat that stalls or nearly stalls), or 2 - the amount of muscular endurance they have THAT IS RELEVANT TO THEIR GOALS. So in a very basic example, "endurance" for powerlifting would be lifting in the 8-15 rep zone whereas that would be a "strength" zone for a marathoner...because their definition of endurance is running for hours on end.
So Periodization is a way to try to make a lego-style building plan for your strength goals or your sport. Basically, look at what is required for your sport--or goal, if it's something like a bench press goal--and look at what you are doing and not doing, and build a plan of attack. It looks different for different goals AND for different weak points (periodization should always address your biggest weaknesses and shore them up to keep you healthy)