Skeuomorphism is the design concept of making an object or feature which imitates the design of a physical object. In the context of computing, it’s an element of a GUI that mimics a physical object. For example, a note-taking app with an icon that looks like a yellow legal pad would be said to be a skeuomorph of a legal pad.
One iconic recent example of this was in iOS 7. That update ditched the realistic-looking icons for a flat, minimalist aesthetic.