the short answer
A pull-up is a body-weight pulling exercise: hang from a bar with an overhand grip and pull your chin above it, mainly using the lats and arms. The single most important cue is to start the pull from your back by driving your elbows down toward your ribs, not by yanking with the arms - lead with the elbows and the back does the work.
This article is general training information for coaches and clients, not medical advice - good form is individual, so a qualified coach giving a live form check is the gold standard, and any pain or injury question belongs with a qualified professional.