Positive displacement would be raised up off of the geometry (like raised letting). Negative displacement would be inset into the geometry (like engraved lettering).

So if your locker door was a flat plane, using a math node as DeathMetalDesk recommended, you could add a dent using negative displacement, and/or add the raised vents that school lockers usually have.

http://www.renderosity.com/mod/tutorial/?tutorial_id=2295 is a good tutorial for learning about displacement

Poser Pro 2012 SR3 on Win10 x64 w/Reality.
Poser Display Units = inches.