The center is considered open if there are no pawns in the center and at least two central files are opened. Positions with an open center are usually characterized by active piece play; both sides try to seize the open cenbtral files and use the central squares in order to transfer their pieces to important sections of the board.
When the center is open, develop quickly, get your king to safety, and don't waste time.
When the center is open, neither player has time to fool around. Attacks can come out of nowhere, for there are no pawns to obstruct the pieces. Fast development is vital; if you opponent completes development before you do, you could be mated before yo could organize a defense. King safety is also crucial. If your king gets caught in the center with no pawns to shield it, you may never reach the middlegame. Before castling, it's especially important not to waste time going for meaningless pawns or threats. Get ready for business first. If you play e-pawn openings, be prepared for open centers. After castling, don't move the pawns in front of your castled king's position. That gives your opponent targets to attack.
Open games are quite tactical and full of sudden turns and surprises. You must be alert at all times, never letting your attention wander.