A bump key set is a specially cut set of keys used to open conventional pin and tumbler locks. Each key fits locks made by a particular manufacturer. A bump key set is a set of keys that fit different types of locks made by one manufacturer, a single type of lock made by different manufacturers, or both. The keys are available online, cut by a key maker, or filed down using any key that fits the lock.
Tumbler and pin locks all work by having spring-loaded tumblers, or pins, that push down onto the blade of key. When the right key is inserted, all of the pins rest on the blade and align with the shear line of the inner and outer cylinder, allowing the cylinder to rotate and open the lock.
A bump key is one that has been specially cut or filed down such that when inserted into the lock, all of the pins are fully extended. The result is a key cut all the way down into a sawtooth pattern, with the valleys spaced such they allow the pins to extend fully. Once inserted and pulled back a notch, the key is struck with the bump hammer or handle of a screwdriver while force is applied in the direction needed to open the lock. This pushes all the pins up above the shear line, allowing the cylinder to turn and the lock to open as the pins fall back down. This method works with the vast majority of home and car locks, as well as for many padlocks.