Example
Buy 100, stop 98, exit 103: you risked 2 and made 3, so +1.5R.
In practice
Measuring in R removes account size and position size from the picture, so trades and strategies can be compared directly. A journal kept in R shows quickly whether winners are large enough to pay for the losers.
It also changes how a loss feels. A full loss is simply minus one, a known and accepted cost, whatever the amount in money.
What to watch for
It only works if the initial risk is fixed before entry and respected. Moving the stop further away during a trade changes what 1R was and quietly distorts every figure.