The term bb/100 is a somewhat technical poker term that players use to describe their success in cash games. It is one way of describing your win rate.
For example, a player might say that their bb/100 is 10, which means they win 10 big blinds for every 100 hands they play. So, in a $1/$2 ring game, this would translate into $20 they make for every 100 hands they play.
The number is a statistical average, so the larger the sample of hands, the closer the bb/100 number will be to the player’s true win rate. A bb/100 calculated over 1,000 hands doesn’t say much, but the same number over 50,000 or 100,000 hands is a pretty good indicator of the kind of win rate one may expect moving forward, barring any major changes. Having a high bb/100 reduces the variances and makes it easier to attack backers if you want to get staked since then you have less chance of going deep into makeup.