The ratio between what you risk and what you aim to gain. Professionals typically look for at least 1:2 RRR.
Example
You go long at €100, stop at €95 (€5 risk), target at €115 (€15 reward). Your risk-reward is 15:5 = 3:1. That means even if only 4 of 10 trades work out, you are profitable — the 4 winners bring 12R, the 6 losers cost 6R. This is why the risk-reward ratio often matters more than your hit rate. Pros filter setups: if R:R is below 1.5:1, the trade is not taken at all.