Mockups is the step taken after wireframing. It is the mixture of wireframe and UI design. A wireframe would give you a basic visual representation of your screen, but the mockup adds the visual aesthetics to it. Mockups are concerned with the fonts, colors, themes and styles of your app or website screens. A mockup is a static screen that can help you understand a prospective final visual representation of your product.
A mockup lean towards a final product but it is not a prototype. In fact, a mockup is the bridge between a wireframe and a prototype.