Big blind
The larger forced bet, one seat left of the small blind. Also the standard unit for stack sizes (bb).
Take a $1/$2 game. The button opens to $6, the small blind folds, and you're in the big blind with $2 already in front of you. You need $4 more to call. The pot holds $9 — the button's $6, your $2, and the small blind's dead $1 — so you're risking $4 to win $9, better than 2 to 1. That price drags hands into play that would be instant folds anywhere else. 9-7 suited, heads-up against a button open, clears it without trouble. The discount is why the big blind sees flop after flop it would otherwise muck.
Stacks get counted in big blinds — "I'm on 80bb" — because that bet is the one everyone at the table has already put up, so it reads the same at any stakes. But the price cuts both ways. Once the small blind folds, you're out of position for the rest of the hand, acting first on every street against a player who acts last. A call that looked cheap before the flop leaks money after it, when the raiser keeps firing and you keep guessing.
The usual mistake is defending on price alone. Beginners see the discount and call with J-4 offsuit or Q-6, then have no plan when the flop misses and the bet lands. Getting the right price to call is not the same as holding a hand that can win the pot once the cards are out.