In
contract bridge, when one
partnership has won one of the two
games needed to win a
rubber, they are
vulnerable. (It's possible for both sides to be vulnerable at the same time.)
The penalties for a vulnerable team failing to make a contract are larger. However, if you win a second game, you get an additional bonus for winning the rubber. See contract bridge scoring for details.
In duplicate bridge, where each hand is scored separately from others, the vulnerable/not vulnerable assignments rotate so that each combination of vulnerability is played equally many times. Each hand is scored as though it occurred in that particular situation.