In poker, a fish is a synonym for donkey and it describes a poker player with below average skills.
The better players will be more likely to make money from the fish at any given table, since the weaker inexperienced players are more likely to be making costly mistakes.
A typical example of a “fish” is a player that loves to chase hands all the way to the river. This happens all the time and drives some players nuts, especially when they were a huge favorite to win the hand and got incredibly unlucky.
Although it’s important to realize that there are different types of fish. For example, weak/tight fish will be folding too much, and loose/passive fish will be playing too many starting hands.