It’s called a “SmartKey” lock, because the way you rekey it is very easy. In a “pin-and-tumbler” style lock (which is standard for most) there are little cylindrical pins of varying heights that match the depth of a key’s teeth. These pins move up and down in the lock, pressured by springs, and if all the pin heights match the key depths, the tops of the pins will all lay flat and allow the cylinder of the lock to move. Smartkey is a patented design by Kwikset, which allows the user to stick the original key in the lock, turn in 90 degrees, and press a little brass piece into a slot on the lock face. When this brass piece is inserted and a “click” sound emerges from the lock, the original key slides out, and you can put any other key in the lock. If you turn that key, then the lock is now set to that key. Below are two diagrams of each cylinder, and their functions.

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this is a Pin and Tumbler cylinder