When you throw something straight up in the air, there is a point where it has no movement at all. Before this point, it is rising, after this point it is falling. But for an instant, it is floating. This is the deadpoint.
(Courtesy of Randall Munroe)
When climbing, you want your body at the deadpoint when grabbing a climbing hold. If you grab before the deadpoint, your own momentum will continue to carry you upward and off the hold. If you grab after the deadpoint, your downward momentum will put additional strain on your fingers, making it harder to hold on. If you grab exactly at the deadpoint, you will maximize your ability to hold on to small holds.