A bulk is a phase in which you build muscle mass and often body fat. During this phase, you supply your body with more energy through calories than it consumes. A caloric surplus is the most important thing during a bulk. With this, combined with smart strength training with steady progression, you can effectively build muscle.
Unfortunately, many strength athletes, most often beginners, misunderstand the bulk. With the wrong principle of "more is more," many people consume more calories than they need to to bulk effectively. A calorie surplus is necessary to build muscle, but it does not need to be too high to build muscle effectively.
With too high a calorie surplus, you end up building more fatty tissue than muscle. An enormously high-calorie surplus is usually called "dirty bulk" or "mass phase."
You eat an extreme amount of food in a dirty bulk to build muscle. You can definitely build muscle this way, but you will also put on a lot of fat. If your goal is to get a defined and trained body while staying lean, then you will have to get rid of this fat in a long diet phase.
The big disadvantage of a dirty bulk is, therefore, the high body fat percentage which is built up in the long term. With an increased body fat percentage, the body also releases more of the hormone estrogen and less of the hormone testosterone. This is not conducive to muscle building.
Furthermore, the higher the body fat percentage, the lower the insulin sensitivity. This means that the body reacts worse to insulin. This is also an undesirable side effect of a dirty bulk if your goal is muscle building.
Even if you don't do a dirty bulk, in which you consume as many calories as possible, but do a bulk with a calorie surplus of about 500 calories, you will build far more fat mass than muscle mass in the long run. Therefore, if you want to stay lean you should go for a lean bulk and try to build your muscle mass as fat-free as possible.
Also, you should train hard and try to progress in each workout to make your muscle-building phase as successful as possible.