Planning training in phases so volume, intensity and exercises change on purpose over weeks and months instead of staying the same.
The vocabulary is layered: a macrocycle is the season or the year, a mesocycle is a block of three to six weeks, a microcycle is a week. Classic linear periodization moves from high volume and low intensity to the opposite; undulating and block models vary that.
For general clients the point is simpler than the names suggest. Change one variable every four to six weeks, plan the deload before the client needs it, and keep a record so you can see which block worked. Even "same exercises, different rep ranges" is periodization.
The alternative, the same programme forever, works for about eight weeks and then stalls, which is where most unsupervised gym goers live.