A muscle's ability to keep producing force over many repetitions or a long hold, rather than to produce a lot of force once.
It is trained with high reps and short rest: sets of 15 to 30 at 50 to 65 percent of 1RM, resting 30 to 60 seconds. That is the opposite prescription from maximal strength, which is why one program cannot maximise both.
It matters more than most trainers program for. Clients who work physical jobs, carry children, hike or play recreational sport are limited by endurance far more often than by their one-rep max.
Test it simply and retest: push-ups to technical failure, a bodyweight squat set to 30, a plank or a dead hang for time. A client going from 12 push-ups to 28 over a block has a result they can feel, which does more for retention than a number on a bar.