bb/100 is the standard way to measure what you win in cash games: big blinds won per 100 hands. Because it's measured in blinds, it lets you compare across stakes: winning 5 bb/100 is the same at NL2 as at NL200.
How it's calculated
How to read your bb/100 (winrate)
Winrate is the most misleading number. Poker variance is huge: with a typical standard deviation of about 90 bb/100, the 95% confidence interval of your winrate is about ±17.6 bb/100 over 10,000 hands, ±7.9 over 50,000 and ±5.6 over 100,000. Over a few thousand hands, winning or losing says very little about how you play.
That's why Flopmortem always shows your winrate with its margin of error and, to find leaks, looks first at your decision stats (VPIP, PFR, 3-bet…), which stabilize with far fewer hands.
The blinds are a special case: the SB and BB lose money almost always, even for the best players. What matters there is losing as little as possible.