A fish is a weak or inexperienced poker player who loses money to better opponents. Typical habits include playing far too many hands, calling bets too often and chasing unlikely draws. The opposite is a shark: a skilled player who profits from those mistakes, which is how the ocean imagery came about.
The word is used mostly at poker tables, but it has spread to everyday speech for anyone easy to take advantage of. An old card-room saying holds that a player who cannot spot the fish at the table is probably the fish. Knowing it may save a newcomer some money, and a little pride.
Why poker rooms protect them
A game made up only of sharks soon dries up. Many online poker rooms protect recreational players with anonymous tables or restrictions on tracking software, which keeps the player pool healthy.