A bump key template is a key or the outline of a key on an index card or paper from which to make a bump key. A bump key is a specially cut key used to open conventional pin and tumbler locks from a particular manufacturer.
A key fits into a pin and tumbler lock when the spring-loaded pins push down onto the grooves of the key such that the pins align with the shear line of the cylinders. When the pins line up in this way, turning the key allows the cylinder to rotate and unlock the door.
Bump keys work by allowing the pins in the lock to extend fully resting on the key blade. By pulling the key back one groove and bumping it with a mallet or screwdriver, the pins jump. With pressure exerted on the key in the direction to open the lock, as the pins fall and line up at the shear line, the key turns, opening the lock.