Definition of snap
An "snap " is a game played at the speed of light. A snap call is a game that is played without thinking "I had the aces preflop and my opponent pushed all in and I have snap call". Usually the word snap is associated with call. There may be some ego in this expression. For example, a player bluffing one on the river and the other one saying "I've got a weak hand, but I know you're bluffing, I don't need to think, I'm standing call in, see how competent I am?
The importance of not snap
Although you may look like a fearsome player when you make a good snap call on the river against a bluff, as a general rule, take your time before you play, even when the game seems obvious. In poker, you're constantly dealing with money at risk and it will be important for you to make the right decisions. You've probably heard players say that they called a all in preflop in "snap call" thinking they had AA when they had A4. A bit of fatigue, a game a bit too fast and you lose a full buy-in .