A coinflip refers to a situation in which two players have almost equal chances of winning. Since it's close to a 50:50 probability, the game may as well be decided by flipping a coin.
Example (Texas Hold'em):
Player A Player B
A classic coinflip in Hold'em happens when a pair before the pre-flop is pitted against two overcards. The chances are around 44:56, with a slight advantage for the queens.