Being the first to raise when no one has entered the pot (an open-raise).
The everyday word at the table; the formal name is RFI (Raise First In). Which hands you open depends on your seat, with later seats wider. Example. Everyone before you folds: on the BTN (the dealer button seat) you open hand A♠ 9♦ (A9o: an ace and a nine of different suits), but from UTG (the first seat to act) the same hand folds. The player who opens is called the opener and keeps the initiative (the role of leading the betting) on later streets.