A deadbolt is a strong lock on a door. Inside it is a solid metal bar, the bolt, that slides into the door frame when you turn a key or a small knob. The spring lock that clicks when you pull a door shut is easier to push back; a deadbolt does not spring. You have to turn it, and then it sits there, which is why it is harder to kick the door in. That is the “dead” part: not “broken,” and not “someone died,” but “not springy — fully thrown and still.” Many front doors have both a light latch and a deadbolt; people who care about safety turn the deadbolt at night. You can also use it as a verb: to deadbolt the door. It is ordinary home English, a little technical if you are in a lock shop, not slang.