A greeble, greeblie or greebly, is the name for the small elements that add detail and scale to models and, often, help make them look more advanced and realistic. Greeble can be both a verb and a noun.
Greeble or greeblie, George Lucas' term, originated on the early Star Wars sets when creating the impressively detailed and realistic models. You can take a simple ship, wall or rooftop, and by adding switches, boxes, cabling, lights and buttons, hey presto, there's a whole lot more technology involved. You can add greebles, or greeblies, to something, or you can greeble it.
A common modelmaking technique for adding elements and greebles to make a design look realistic and sophisticated is to scavenge from existing model kits. Borrowing other parts and repurposing them on a new model is called kit-bashing. Speaking personally, it's also very satisfying making up greebles to add to a LEGO build (MOC—My-Own-Creation)—sometimes the most fun part.
Here's Adam Savage, an ex-Star Wars modelmaker discussing greeblies and kit-bashing (video) or an ILM (Industrial Light and Magic) model shop greeblie (video) used on Star Wars Episode 2. Greebles also make an appearance in the fascinating documentaries about the making of the Star Wars films, Empire of Dreams, and the making of episode 9, The Skywalker Legacy. Sometimes, the making-of is more interesting than what was made. I remember the same feeling watching the making of the Lord of the Rings films, too.