the short answer
Progressive overload is gradually increasing the demands on a muscle over time so it keeps adapting and getting stronger or bigger. Coaches apply it by adding load, reps, sets, frequency, or range of motion. Without a steady rise in demand, the body adapts and progress stalls.
This article is general training information for coaches, not medical advice - individual capacity, injury history, and recovery vary, so program within each client's tolerance and refer clinical questions to a qualified professional.