Simply stated, periodization is a systematic build of fitness, divided into areas of specific training, with the goal of reaching a fitness peak at a specific time. 

This graph shows a typical 4 week periodization cycle over a period of 16 weeks.  Ie. Cycle 1 = 4 weeks of progressive build with a rest week on the last week.  

© National Strength & Conditioning Association Volume 25, Number 6, page 19–37