A win rate in poker is a number that shows how profitable a poker player is in the games they play.
This number is usually expressed as the ratio of big blinds won over 100 hands (bb/100) or money won per hour. The first number is usually used in the online setting, while the latter is more common for live players.
For example, an online grinder might say that their win rate is 3BB/100. This means that, on average, they win 3 big blinds for every 100 hands played. Live players may express their win rate as $20 an hour, meaning they win $20 for every hour spent at the table. Win rate can also be measured in terms of ROI for tournaments.
Either way, a win rate is important to give a player an idea of what they can expect to win playing poker over the long run, allowing them to better plan their finances and their future career.