Definition of snap
A "snap" is a play that you make at the speed of light. A snap call is a play that you make without thinking "I had aces preflop and my opponent shoved all in and I snap called him". Generally, the word snap is associated with call. There can be ego in this expression. For example, one player bluffs on the river and the other says "I snap call" as if to say "I have a weak hand, but I know you're bluffing, I don't need to think, I'll call instantly, see how good I am?
The importance of not snapping
Although you may look like a formidable player when you make a good snap call on the river against a bluff, as a general rule, take your time before playing, even when the game seems obvious. In poker, you have money on the line all the time and it will be important for you to make the right decisions. You've probably heard players say that they called a preflop all in as a "snap call" thinking they had AA when they had A4. A little bit of fatigue, a little too fast play and you lose a whole buy-in.