A bulk and cut cycle is the planned alternation between building phases with a calorie surplus and diet phases with a deficit. In the bulk you build muscle and accept some fat, in the cut you lose that fat again while keeping the muscle.
The thinking behind it: building muscle is easier in a surplus, losing fat is easier in a deficit. Instead of chasing both at once, you separate the goals in time. For intermediate and advanced lifters who are already lean, this usually works better than a recomp.
Where to start
Whether you begin with a bulk or a cut depends mostly on your body fat percentage. As a rough rule of thumb from practice, not from studies: men above about 20 percent tend to start with a cut, men below about 15 percent with a bulk, and for women the values are roughly 8 to 10 percentage points higher. In between, your FFMI, years of training and goals help you decide.
When to switch
There is no fixed length. A bulk usually ends when body fat has risen to the point where you no longer feel comfortable or the next cut would get too long. A cut is better ended at a set date or target weight than when motivation runs out.
Smaller blocks fit between the big phases: a mini cut if the bulk brought too much fat, or a stretch at maintenance between two building blocks.
The rate keeps the cycle short
The slower the bulk, the less fat you have to lose afterwards. Iraki et al. 2019 point out that too much fat gained in the off-season makes the following diet longer or harder. Whether you are on track shows in the weekly rate of your trend weight.