This poker term means to open the betting by coming in with a raise. There are forced bets before the flop, so you can open the action by making a raise to put pressure on the blinds.
For example, you are playing a $1/$2 NL Hold’em live cash game and get dealt A-Q suited, and open-raise to $12 in MP and get called by the BTN and everyone else folds.
Typically, open-raises are larger in live poker games. It is common to see 6x, 8x and even 10x BB pre-flop raises which get calls.
Aggression is a key factor to winning at online/live poker, and this means raising when opening the action in most situations.