Wireframe, Mock-up & Prototype
A wireframe is a deliberately grey structural draft that settles a page's content and order; a mock-up shows the same page fully designed with colours, typefaces and images; a prototype links several screens clickably so a flow can be tested before any code is written.
In practice
The sequence is the real benefit: as long as only greys are visible, everyone discusses content and priority, but the moment colours appear the conversation turns to taste. Use this phase to settle uncomfortable questions: what must a visitor do on this page, and what stands in the way? A clickable prototype is especially worthwhile for forms, configurators and booking flows, where flaws can be fixed in the design at a fraction of the cost. Show the prototype to two or three people outside the project team; that typically reveals more than another internal review. Do not mistake a prototype for a finished website: it looks functional but has no database, no integrations and no real processing behind it.