Body recomposition, or recomp for short, means building muscle and losing body fat at the same time while your weight stays roughly the same. Instead of alternating between bulking and cutting, you try to do both at once.
Part of the energy for building muscle comes from your fat stores. Because the two effects roughly cancel out, the scale barely moves. Barakat et al. 2020 note in their review that there is no single scientific term for this, and name progressive strength training and sound nutrition as the main levers.
Who it suits
It works best for beginners, people returning after a break and people with a bit more body fat. According to Barakat et al. it is also possible for trained lifters, but the effects are smaller. If you are very lean and have trained for years, bulking and cutting usually gets you further, faster.
The requirements are progressive training, plenty of protein and calories at maintenance or slightly below. A range of 1.6 to 2.2 g of protein per kilogram of body weight is a sensible starting point. A large deficit works against you, because it is more likely to cost muscle.
How to track it
A flat trend is the expected result of a recomp, not a stall. That is why you need other measurements: waist size once a week, photos under the same conditions and your strength numbers. If your waist shrinks at the same weight while your lifts go up, the recomp is working. If your trend climbs week after week, you are eating above maintenance.
Give it at least eight to twelve weeks before you judge, because waist and photos change slowly.