The term comes from "flipping a coin". When playing Hold 'Em, a flip can occur, for example, when one player has a pair and another player has overcards. A flip can also be known as a "toss up" or a "horserace".
Two cards, known as hole cards, are dealt face down to each player, and then five community cards are dealt face up in three stages. The stages consist of a series of three cards ("the flop"), later an additional single card ("the turn" or "fourth street"), and a final card ("the river" or "fifth street").