After all players have been dealt their hole cards and the first round of betting is complete, the dealer will discard the top card of the deck and deal three cards into the centre of the table.
The process of discarding the top card is done to prevent cheating and to ensure no one might have seen the top card before the deal. This is called "burning a card". Obviously in an online setting this is not a problem but the practice continues in the name of tradition.
The three face-up cards in the centre of the table are known as the flop. Because these are face up cards they are referred to as community cards because everyone can use them to make a hand.
After the flop has been dealt, the players remaining in the hand make another round of bets. Again, the betting always begins with the small blind, or if he is out of the hand, the next player to his left.
Players can either check, call, raise, re-raise or fold according their turn to bet. Once this sequence has been completed, it's time to go move on to the Turn.