A desktop is the surface of a desk, the part you use for writing or resting your laptop on. Particularly tidy people might need to clean the entire desktop before getting any work done.
Your literal desktop — the top or surface of your desk — inspired the name for your computer desktop, the home screen on which you organize icons representing your files. When you're using a computer, you generally start from your desktop, opening windows or clicking on a file. When desktop is used as an adjective, it typically means "meant to be kept and used on your desk," like a desktop computer or a desktop scanner.