the short answer
Muscular endurance is the ability of a muscle or muscle group to repeat or sustain contractions against a sub-maximal load without fatiguing, for example how many push-ups a client can do in one set or how long they can hold a plank. Where maximal strength asks how much you can lift once, endurance asks how many reps you can complete before the muscle gives out. You train it with lighter loads, 15 or more reps and short rest.
Below: a table of endurance vs strength vs power vs hypertrophy with typical reps, load and rest, a five-step way to program an endurance block, and three tests you can re-run every four to six weeks.
This article is general programming information for coaches, not medical advice - load, rep ranges, and testing should always be tailored to the individual client and their health history.