Flop
The first three community cards, dealt face up together. Where most hands take shape.
The flop is the first real information in the hand: three cards turned up together, right after the dealer burns one face down. Before it, you have two hole cards and a guess. Say you raise from the cutoff with 8♠7♠ and the button calls. The flop comes 9♠6♠2♦. Preflop, 8-7 suited was a speculative hand; now it's an open-ended straight flush draw — 15 outs, since any five, ten, or spade fills it. That one card moved the hand further than the whole preflop round did.
This is why most pots get decided here, long before the money goes in. Queens look huge preflop, then shrink to a bluff-catcher when the flop lands king-high and connected. A weak ace can flop two pair. Read the texture: wet boards full of straight and flush draws, dry boards with almost none. It tells you how hard to bet for value and how much to respect a raise coming back at you.
The beginner mistake is treating the flop as confirmation of the preflop plan. A player calls a raise with king-jack, sees a queen-high, disconnected board, and keeps calling because the hand still looks fine — never noticing they now hold one overcard, a backdoor draw at best, against a range stacked with top pair or better. The flop resets the math. Read it fresh instead of coasting on the preflop story.