It typically refers to a situation where two players are all-in, and the outcome of the hand is largely determined by chance or luck. The term is often used in the context of a coin flip, indicating that the players have relatively equal chances of winning the hand. Flips commonly occur when both players have hands with similar equity, and the outcome is uncertain until the community cards are revealed.
It's a colloquial term and is sometimes associated with situations where the skill element of the game is temporarily set aside in favor of luck.