Definition
Dividing training into blocks of three to six weeks, each with one main goal, such as volume, then strength, then a peak.
The blocks have names: accumulation (volume, 65 to 75%), intensification (75 to 90% with less volume) and realisation (the peak, low volume and high intensity), usually followed by a deload. A common split is four, three and two weeks plus one.
Concentrating one stimulus at a time produces bigger adaptations in advanced lifters, and it is the standard way to bring a strength athlete to a date. For beginners it is more structure than they need.
Fatigue from the accumulation block is meant to carry into the next one, which is why the deload at the end is not optional.