RIR is how many more reps you had left in the tank at the end of a set.
Two reps in reserve means you stopped with two more in you. Zero means you could not have done another one. Coaches use it because the same weight is a different job on different days: 100 pounds with four reps left is easy work, and the same 100 pounds with nothing left is close to your limit. RIR describes the effort, not the load.
Reasonably well close to failure, and less well the further away they are. The scale comes from Zourdos and colleagues in 2016, and the research that followed finds trained lifters estimate it most accurately on hard sets and get vaguer on easy ones (review). So it is a useful number when the set was hard and a noisy one when it was not — worth knowing before you build a whole programme on it.
Rhythm does not ask you for RIR today. It reads plateaus from the weight and the reps you actually logged, which are facts rather than estimates.
The cost of that choice is real: Rhythm misses the sign where effort climbs while the weight stays the same. That is why the app watches two signals rather than three, and why we say so on the page about deload signs rather than letting you assume it sees everything.