WHAT IT IS
The regular practice of changing things up in your training program—using different exercises, set and rep schemes, rest periods, and modes of activity. Even changing seemingly small variables like grip or the kind of bar used for a particular exercise can help contribute to muscle confusion.
WHAT IT DOES
For lack of a better phrase, it "keeps your body guessing." When you do the same exercises with the same sets and reps month after month, year after year, the muscles adapt to the predictable stimulus and cease to grow bigger or stronger. By changing training variables, your body is challenged to keep up with the demands placed upon it, and has no choice but to change.
HOW TO USE IT
Most periodized programs follow a particular phase of training for around 4–12 weeks before switching gears. For example, a hypertrophy phase followed by a strength phase, where exercises, volume, and intensity are altered. Change your program every 4–8 weeks but always stick with it long enough (at least a month) to see if it's effective.