A fish is a player who doesn't have an strategic understanding of the game. Of course depending on which level we are, we could be the fish to another more skilled player.
Of course the fishest of fish is the player who has never studied the game, and the term is often used o refer exclusively to this type of player. Identifying a fish is easy. If you see someone open limp the pot there's a 99.99% chance they are a fish, if you see them do it a second time then that's 100%. Someone who is playing above 40% percent of hands with 5+ players in the table is also probably a fish. Fish just doesn't know a thing about ranges, they will look at K9o and call an UTG raise, if they flop a pair they'l call it all the way down to showdown, unless the amount has become too big for them to feel comfortable. Beacuse for them it's all bout the feel. Since they lack knowledge it's all about whether they like a hand or not.
There are passive fish, aggro fish, maniacs, calling stations, they all have different tendencies, that are easyto exploit. Value bet calling tations relenlessly with you strong hands, induce aggro fish to
bluff, c-bet fit or fold players, go aggro on nitty players and so on