Progression is a player's ratings improving over time, and regression is their decline. In a basketball sim both follow an age curve: young players grow, players in their prime hold, and veterans fade.
In HoopSim young players who get real minutes improve a little at a time during the season, and every player gets a bigger development pass at season's end that weighs age, playing time, your coaching staff and how they were used. Players grow most in their late teens and early twenties, peak in their late twenties, and decline in their thirties, quickness, leaping and stamina first. What a player does on the floor shapes which skills grow, a head coach's specialties push growth in their areas, and training camp lets you steer each player once a year. Growth is not even: breakouts, late blooms, slumps and sudden drop-offs happen and make the league feed. The commissioner can adjust how steep the curves are.
Rulebook: The age curve. Related: potential, OVR.