Just about everyone who's been training for a while has heard of periodization. However like many other things, there is a ton of misinformation about it. There are of course many different methods and possible training cycles that can be used. Far too many to list here, but I want to provide a basic overview of what it's all about.
So to put things simply, periodization is all about the fact that you can't train at the same level all the time. Back when I was first learning about working out, the gym rats I was working out with had no idea about periodization or cycling as it's often called. They would just go and go, then they would eat and eat! I followed suit and it does work for a while. Eventually however you will reach a plateau and you will no longer be able to get any more reps or weight. So what do you do? You need to change the program by going lighter, faster pace, fewer reps, more reps, etc. Things must be changed. I learned of the concept of cycling from reading about it and tried it out. It did work, but again only for a short period of time. However my first real introduction to cycling and my first real success with it came from my training in powerlifting.
In powerlifting, there are three lifts. The Squat, the Bench Press, and the Deadlift. My personal best was the Deadlift. Then you have assistance exercises. We would generally train in 12 week cycles. Sometimes longer. The idea is to take about 60% to 70% of your current best weight (whether it's for a 1 rep max or multiple reps). Then you want to do a few sets of perhaps 10 reps. Even if you can do more, don't. The idea here is to train lighter and give your body and mind a chance to recover. Over the course of the cycle, the weight is gradually increased until you blast past the previous best weight and hopefully end up peaking on the day of the meet. Of course this doesn't apply if you're not a competitive lifter, but the same structure can be used.
This is the basic method of cycling and it works great. Within this structure there are countless possibilities. Another way of using periodization and one which I often use with clients is to have different levels of intensity ongoing. Monday could be a hard weight day, Wednesday could be a more cardio based HIIT day, and Friday could be some conditioning and Boxing. These are just some examples and again there are countless possibilities.
So the big takeaway with all this is to VARY YOUR INTENSITY! Whether it's set up in cycles or it's ongoing it works. Cycles can also be combined with ongoing periodization as well and it can get pretty complex. But it's always best to just keep things as simple as possible. By following periodization, methods, you can prevent injury, staleness, boredom, AND make better progress!