Preflop
The betting round before the flop, on your two hole cards alone.
Preflop covers everything before the flop hits the felt: blinds post, everyone gets two hole cards face down, and action starts with the player left of the big blind. Say you're on the button with A-J suited. Under the gun opens to three big blinds with pocket queens, two players fold to you, you call, the small blind folds, and the big blind calls. Three of you see a flop of J-8-4 rainbow. Everything before that first card happened preflop, the one street where your two cards are all you've got.
Position decides how wide you play. K-9 offsuit is an easy fold under the gun and a fine open from the cutoff or button, where you act last on every street that follows. Get preflop wrong and you leak chips two ways: limping in with junk, or folding hands that would make money in position.
The beginner mistake is playing every seat the same — calling raises with weak offsuit cards because it's only one more bet, limping instead of raising a hand worth a raise. Preflop is the cheapest street to lose chips on and the cheapest to fix. Fold more from early position, raise more from late, and let your two cards do the talking before the flop falls.