The process of building new muscle protein, which rises after a training session and stays elevated for roughly one to two days.
Muscle grows when synthesis exceeds breakdown over time. Training raises synthesis; so does eating protein. Neither on its own does much, which is why a client who trains hard and eats 40 g of protein a day makes little progress.
The response to a session peaks in the first few hours and is largely finished by 24 to 48 hours in trained lifters. That is the physiological argument for training a muscle twice a week rather than once: the second session restarts the response before it has fully settled.
The anabolic window is smaller than it was sold as. Total daily protein matters far more than timing, spread over three or four meals across the day.