In the fitness community, "Periodization" refers to varying your training volume and intensity in accordance with goals for a particular period of the year. In the video below, Coach Hansen explains how an elite junior golfer can use periodization to be able to maximize gains and achieve peak performance when it's needed. (You can stop here or I would add the following:
Periodization is not just as simple as in-season and off-season. Instead, there are four different phases of the year:
Pre-Season
In-Season
Post-Season
Off-Season