Open-raise
The first raise before the flop, when nobody has entered yet.
Nine-handed, blinds posted, everyone folds to the cutoff with king-nine suited. No raise yet — the blinds are forced money, not a decision, so the cutoff makes it three big blinds. That's the open. Button folds, small blind folds, big blind calls with king-jack. Heads-up to the flop, and the cutoff set the terms.
Raising first matters because you set the price, and plenty of pots end the second everyone folds to it. It also decides which hands are worth playing from each seat. Under the gun has eight players left to act, so you need a real hand to open — a good pair, ace-king, ace-queen, the better suited broadways. The button has two players left, the blinds, so king-nine suited or worse shows a profit on fold equity and position alone. Same three big blinds, different math.
Two beginner leaks. One is running the same range from every seat — opening nine-two offsuit under the gun because it printed from the button last orbit. The other is limping a hand worth raising. Calling the big blind hands the initiative back and lets the whole table in for one bet. Open-raising puts money in first and makes everyone else answer it.