the short answer
Rep ranges are the bands of repetitions you do per set to bias a goal: roughly 1 to 6 reps for maximal strength, 6 to 15 for hypertrophy, and 15 or more for muscular endurance. The key nuance is that all of these ranges build muscle when sets are taken close to failure with enough weekly volume.
This article is general training information for coaches, not medical advice - individual response varies, and clients with injuries or health conditions should clear new training with a qualified professional.