A rep range is the span of repetitions you aim for in each set of an exercise, for example 8 to 12. Instead of a fixed number, it gives you a corridor within which you can progress from session to session.
Which range for muscle growth
For building muscle, the range of workable loads is wide. Schoenfeld and colleagues (2017) compared light and heavy loads and found similar muscle growth when sets were taken close to failure. For maximal strength, heavy loads came out ahead. A systematic review (Baz-Valle et al. 2021) treats sets of roughly 6 to 20 or more reps as equivalent, as long as they are hard enough.
So the exact number matters less than how close you get to failure. In sets of up to 12 reps you usually judge your reserve more accurately. In long sets of 15 or more, you tend to underestimate it.
Double progression
A rep range is the basis of double progression. You start with a weight that lets you hit the bottom of the range in all sets and then add reps. Once you reach the top of the range in every set, you add the smallest sensible weight increment and start again at the bottom.
Example: Range 8 to 12, bench press with 60 kg. When you get 12 reps in all three sets, you move to 62.5 kg and work your way up from 8 reps again. This is an example.
That way every session has a clear plan, without complicated percentage math.