A snap call describes a situation in which a player calls a bet from another player immediately, without any thinking or hesitation.
For example, you could say: My opponent bet full pot on the river, and I snap called them because I knew they were on a flush draw and the river bricked.
When a player snap calls, this usually means they have already made up their mind about what action they’ll take if their opponent bets, and they have nothing else to think about.
Although it can look pretty cool, there is no particular strategic advantage to snap call a bet, especially on the river, when there is no future action to come. Taking a few moments to double-check the bet size, the previous action, and your hole cards (if necessary) is often advisable.