RIR is how many more reps you feel you could have done past what you actually logged — 0 means you hit true failure, 5 means the set felt easy. On a set where it's asked for, the active workout card shows a row of buttons after you log your reps: pick 0 through 5, or tap — if you'd rather skip it for that set.
RIR is logged purely for your own reference — to look back on later and see how hard a session actually felt, not just what the numbers say. The progressive-overload engine never reads it: entering a number, changing your mind, or leaving it as — has no effect on your targets. Only the reps and weight you log drive advancement. Whether you're prompted for RIR at all is a preference you can turn on or off globally, or override per exercise.
On the watch, the same prompt appears as its own screen right after you log reps for a set that asks for it.
Related: "Logging a workout", "How progressive overload works"