Lockers are a permanent item storage, similar to Containers.
Items placed in a player's locker will remain there across player deaths, remorts, copyovers, and even game crashes! One locker can hold the items of many different players, all separately, and an item put in a locker will be accessible from any other locker in the world, by that locker's owner. (So if you put an item in Locker A, then travel to Locker B, that item in A will also be available in Locker B until you take it out.)
A player's locker starts with a weight and item capacity of zero -- by using the 'lockercreate' command, they can destroy a container they are carrying to set the locker's limits to the same as that container.
They are also available to be purchased for use in Player Housing.