An R-multiple measures the result of a trade in terms of the risk you originally took. Your initial risk — the distance from entry to stop-loss, multiplied by position size — is "1R". A trade that makes twice what you risked is +2R; one that hits your stop is −1R.
Why professionals think in R
Thinking in R-multiples separates the quality of a strategy from the size of your account. It lets you compare trades on different stocks at different prices on equal footing, and it enforces consistent position sizing: risk the same small percentage of capital (say 0.5%) on every trade, and no single loss can hurt you. A strategy can win less than half its trades and still be highly profitable if its average win is a larger R-multiple than its average loss.