Definition of flop
The flop is the first part of the community card table. In Hold'em, for example, each player is dealt two private cards that belong to him. He can use them together with the 5 community cards, which will form what is called the board. This 5-card board will be divided into three parts: part number 1, 3 cards revealed at the same time, called the flop. Part number 2, one card called the turn. Part number 3, a fifth card that we call the river.
The flop, the place where (almost) everything is played
In total, a Hold'em player will be able to use 7 cards to make a hand; his two hole cards and the 5 community cards. At the 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 shows AKJ all in hearts, I know that 99.99% of the time 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 get JJ, I know I can go on.