The term preflop is used in variants of poker that include community cards and have a flop. It refers (logically enough) to that portion of the hand which is played prior to the dealing of the flop.
Nearly always, the term preflop is used to describe the initial stage of play in a game of Hold 'em or Omaha, the two most common community card variants. The other stages of play are the betting on the flop, on the turn, and on the river.
Preflop play is critical in two ways:
Starting hand selection[]
One of the most critical decisions you must make in playing Hold 'em or Omaha is whether you will enter the pot at all with your initial hand. This decision is called starting hand selection, and is one of the top three factors influencing a player's profit-making potential. In general, selecting strong starting hands and discarding all others is the most profitable way to play, but recognizing which hands are strong in which positions for which reasons can take thousands of hands of experience to learn. A usefull tool for beginners to learn pre flop play are online tools like the one from Preflopper.com
Defining your hand[]
Your preflop play is where you begin to define your hand to the table. If you raise or even re-raise preflop, you are declaring to the other players that you hold a very strong hand. If you call, you are generally declaring that your hand is weak to middle strength. If you cold call, you are declaring that your hand is of middle-to-high strength. If you raise a large amount (or, in limit poker, raise to the cap), you are declaring you hold one of the most powerful possible hands.
Whether your declaration is true is another matter - you may be slow playing or bluffing. But a successful bluff always starts with preflop play, since a bluff requires that you "tell a story": you must convince the other players that you hold a specific hand. In order to do that, you must play your cards preflop as if you actually held the cards you are trying to represent.