In the Lego online community, greeblies are the intricate details on the sides of spaceships and sci-fi MOCs that define the mechanics and workings of what the MOC is meant to be. They were famously used in
the giant Star
Destroyer Lego set a few years ago, and also in the more recent
Death Star II set. Plates with rails are often used for greeblies because they have a protrusion that only sticks out half a stud without needing a jumper plate. (source: Brickspace.org)