Ok by no means am I an expert so I'll keep it really ELI5 level:

It's very difficult to turn fat into lean mass, known as body re-composition. It's generally thought the easiest way to build lean mass is to eat at a surplus while lifting a lot, but you will also get fat. This is bulking and can be done as a dirty bulk - eat all the things - or a clean bulk - eat lean protein and a good diet, just a lot of it. Cutting is eating at a deficit, but continuing to lift and eating a high protein diet with the goal of retaining as much of the lean mass as possible. Repeat.

I like the rule of thumb that you should 'bulk until you hate how you look, and cut until you hate how you look' but its pretty vain and I keep it to myself.