How many clean reps you had left at the end of a set. RIR 2 means two more were available.
RIR is the flip side of RPE: RIR 2 equals RPE 8. The reason it caught on is an uncomfortable one — most hypertrophy stimulus comes from the last reps before failure, yet going to true failure every set costs more recovery than it returns. RIR 1–3 hits that window. Practical cue: once your rep speed visibly drops and your back starts helping, you were already at RIR 0 — even if the weight still moved.