Training breaks muscle down. Recovery is what builds it back up, stronger than before — and that second half of the process gets skipped in a lot of workout plans that only focus on the exercises themselves.
When you lift a weight heavy enough to challenge you, you create microscopic damage in the muscle fibers involved. In the 24 to 72 hours afterward, your body repairs that damage and adds a bit extra, a process called supercompensation. This is why strength and size gains show up between workouts, not during them. Training again before that repair finishes — hitting the same muscle group hard two days in a row, for example — interrupts the process instead of adding to it.
Three things drive recovery more than anything else: sleep, protein, and time. Sleep is when the majority of muscle-repairing growth hormone is released, which is why chronic short sleep blunts training results even if the workouts themselves are solid. Protein supplies the amino acids used to rebuild fiber — spreading roughly 20 to 40 grams across several meals a day covers most people's needs. And time simply means giving a trained muscle group 48 hours or more before loading it hard again.
Soreness (DOMS) is a rough proxy for how much a session challenged you, but it's not a requirement for progress, and it fades as your body adapts to a given exercise. Chasing soreness as a goal usually just leads to overtraining rather than better results.