The abbreviation 'bb' refers to big blinds which is the standard way of measuring stack sizes and bet sizes in poker. It's fairly typical for a poker player to buy into a cash game with 100bb (one hundred big blinds). In a $1/$2 poker game (the big blind is $2) a 100bb buy-in would be $200.

Big Blind  (BB)  Two to the left of the Button, pays the big blind. Small Blind  (SB)  One to the left of the Button, pays the small blind. Button  (BTN)  To the right of the blinds, the best position at the table.