A one timer is a type of hockey shot that is executed by a player who receives a pass and shoots the puck immediately, without settling or stopping the puck before releasing his shot. One-timers get their name from the "all-at-one-time" motion that defines them. Typically, a one timer is taken with a slapshot or snapshot. One-timer shots are very effective but they require a fairly high degree of hand-eye coordination.
Timing the one-timer with precision and accuracy is what makes it so difficult to perform. It's fairly easy to shoot the puck when you already have possession of it and you have the opportunity to settle it before releasing your shot. One-timers require you to judge the speed of the puck and the angle at which it's approaching your stick, while simultaneously maintaining your awareness of the goaltender's position in the net and the location of nearby defenders.
It's quite common for players who are just learning how to take a one-timer to fan on their shot or to have poor aim when releasing their shot, but it is a skill worth practicing because there are many benefits. One-timer shots are often harder and faster because of the compounded velocity of the pass and the shot. In addition, when a shooter is able to release the puck quickly, as is done with a one-timer, it gives the goaltender very little time to get set and square up to the shot.