In strength and conditioning, specifically with athletes the idea of periodization is behind all programming. It is the idea of manipulating training variables like intensity, technique, volume, and frequency to allow for the athlete to perform at their peak when their competition or season arrives.
Periodization in fitness is different because most people don’t have deadlines or competition dates (some do!). It involves varying the individuals program to cycle through training the different body systems allowing for maximal efficiency and variety.
Periodization, when used effectively, will allow for continual growth in strength, cardiovascular and muscular endurance, mobility, and body composition on a long-term basis. It achieves this through short-term focus on a specific training component, and shifting in due time while maintaining the progress made in the previous training block.
Each time you shift your training focus, your body has learned and adapted to that stimulus. This means that when you return to training in that manner, your body responds quicker each time. This allows for consistent, if not always linear, progress.
For example, if you build strength for 2 months, then shift your training to focus to muscular endurance, but continue to maintain a semblance of that strength, you will lose some, but not all and you will find yourself back to your previous strength and more much quicker when you do return to strength training.