It is calculated as winning trades divided by total trades. If thirty of fifty trades were profitable, the win rate is 60%. It is easy to compute and easy to feel, which is why it gets more attention than it deserves.
A high win rate can sit beside a negative expectancy. A trader who wins 70% of trades with an average win of 0.4R and an average loss of 1.2R has an expectancy of 0.7 × 0.4 − 0.3 × 1.2, which is −0.08R per trade. A low win rate can be profitable: winning 35% of trades at 2.4R against losses of 1R gives +0.19R.
Win rate does matter in two ways. Psychologically, a low win rate means long losing runs that must be sat through, which many people find harder than the arithmetic suggests. And it constrains which payoff profiles are workable: a method with a very low win rate needs reliably large winners.
It also moves with behaviour. Cutting winners short raises the win rate and shrinks the average win. That can feel like improvement while the expectancy falls.
Report it with the average win and loss, never alone.