the short answer
Hypertrophy training is resistance training designed to increase muscle size. It works by driving growth through mechanical tension, sufficient weekly training volume, and progressive overload - usually with sets of about 6 to 15 reps taken close to failure. The goal is bigger muscles rather than maximal strength or endurance, though strength improves alongside it.
This article is general training information for coaches, not medical advice. Adjust volume and load to each client's experience and recovery, and refer pain or medical concerns to a qualified clinician.