Definition
How far a joint moves during an exercise, from the start position to the end and back.
Full range of motion, especially at long muscle lengths, tends to build more muscle than partials. Partials still have uses: overloading a strong angle, working around pain, or matching a sport.
Range is often limited by control rather than by tissue, so a client who cannot squat deep with a bar may squat deep holding a post.
Standardise it. The same depth on every rep, every week, or the numbers in the log are not comparable. A deeper squat at the same weight is a real form of progressive overload.