Snap, sometimes also called jounce, is the technical term for a change in the change of the rate of an object's velocity.
Imagine that you are in an elevator moving smoothly upwards; the speed that it is moving is called its velocity. If it starts to speed up, this is its acceleration. Both of these are common terms that we understand intuitively even if we do not know the terms; sometimes things move, and sometimes they change speed.
But we also might notice when the elevator speeds up, and then suddenly jumps forward -- this is a change in the acceleration, known as the jerk. I am not making this up; this is what happens when scientists don't have a classical grounding in Latin. A snap is the next step up -- a change in the change in the acceleration. This is about the maximum of change that the average human can effectively process; at or immediately after this point, most people would simply say the ride is jerky, uneven, or simply nauseating. In order to capture this feeling, later changes in changes of acceleration have names of crackle and pop.
Another example of this would be jumping on a trampoline; when you are at the top of a jump your acceleration follows a smooth curve, 9.8 m/s/s, acted upon only by gravity (all later derivatives -- jerk and snap -- are zero). When your feet first touch the trampoline your acceleration changes; you are still moving downwards, but now there is a second factor influencing your acceleration -- negatively -- and this is measured as jerk. Then, if things go horrifically wrong and the trampoline bottoms out as your feet hit the floor, your acceleration changes again, and the sudden snap breaks your legs.
Of course, a more complex and less painful example would be a roller coaster, in which acceleration changes over multiple axis many times; the ride's twists and jerks and drops are primarily jerk, but snap adds a little bit of extra fun. However, combining forces in this way can be dangerous, so ride designers have to be very careful not to have too many forces acting in too many directions all at once.
Jerk and snap are most likely to be a matter of concern when things are moving fast or strong forces are involved, as in rocket launches, cams and pistons, and pretty much any equipment in the machine shop. Vibration is a indicator that jerk and snap are present, as acceleration itself should not cause any vibration -- a constant load does not interfere with itself, and therefore there are no interference waves.
Just as acceleration is symbolized as 'a' in equations, and jerk is symbolized 'j', snap is symbolized 's'.
Source: IOPScience.iop.org: European Journal of Physics: Beyond velocity and acceleration: jerk, snap and higher derivatives by David Eager, Ann-Marie Pendrill, and Nina Reistad, published 13 October 2016.