Fish
A "fish" is a weak or inexperienced poker player who loses money over time by making basic mistakes, like playing too many hands or chasing draws.
The word describes a player who is easy to beat because they make predictable errors: calling too often, betting the wrong amounts, ignoring position, and staying in pots they should give up. The term comes from the idea that stronger players "feed" on them. It is the opposite of a shark, and it overlaps with a fun player or a recreational player.
You will hear it most in casual table talk and online chat, usually when someone is sizing up the lineup. Note that it can be condescending or rude when aimed at a person, so at Wit's Games we use it to describe a style of play, not to put anyone down. It is roughly the opposite of a nit, who plays far too tight rather than far too loose.
For a beginner, the useful takeaway is not the insult but the lesson: the habits that get someone called a fish (playing too many hands, calling without good odds, and not folding when beaten) are exactly the ones to drop early. Tighten up your starting hands, pay attention to position, and fold when the math says to, and you avoid the leaks the label points at.
Example
A player limps in from under the gun with 7♣2♦, the worst starting hand from the earliest position, then calls a raise to see the flop. On a Q♥-8♦-3♣-K♠-J♦ board their 7♣2♦ never connects (no pair, no straight, no flush), yet they keep calling all the way to the river, paying off every bet with nothing. That call-anything, fold-nothing style is what gets someone called a fish.