Definition of flop
The flop is the first part of the common maps table. On Hold’em, for example, each player is dealt two private cards that belong to them. They can use them in conjunction with the 5 community cards that make up what is called the tableau (the board). This 5-card table is divided into three parts: part number 1, 3 cards revealed at the same time as the flop. Part number 2, a card called the turn. Part number 3, a fifth card called the river.
The flop, the place where (almost) everything is played.
In total, a player of Hold’em can use 7 cards to make a hand; his two hole cards and the 5 community cards. At flop, the player will know 5 of the 7 possible cards, or 70% of the cards he has to know. This is where he will make most of his decisions. If I have a hand like 56 in spades and the flop hand gives AKJ all the way to the heart, I know that 99.99% I won't have the best hand. I know that no matter what combination of turn and river is possible, my hand will be weak. And conversely, if on the same flop I have JJ, I know I can keep going.