An air lock in a space craft is a door used to move between the sealed environment of a space craft to the vacuum of space (or other craft) with minimal impact to the sealed environment.
I always think of airlocks on space craft, like cabin doors in the Alaskan bush, never physically locked. If you have managed to get to the door alive, and you want in, come on in.
Are any airlocks actually locked (circa 2016)?