How do you explain what a "desktop" is to a completely, non computer savvy person?

A desktop is what you see when you first turn the computer on. Tell him to get back there.

I would just say, “look at your screen” and leave it at that. I also like how @Imadethisupwithnoforethought worded it.

Tell them it is a metaphor of the computer screen as a virtual desktop.

It is a virtual place on your computer that has shortcuts to the things he uses most.
Some people have messy ones, some have uncluttered ones.
The quickest way to show your desktop is to Hold the ⊞ Win + D
(⊞ Win + M brings your Windows back)

Screensaver: book cover

Desktop Icons: book chapters

I would say it’s the home or main page…assuming he’s seen some kind of central HUB interface.

Compare it to the dashboard of a car plus all the buttons, levers, gear shift, brake pedal and so forth. The computer’s engine consists of all the programs in the background like transferring data from hard drives to main memory…

You should have seen me explaining computers to the ladies in my office 20 years ago. I had to explain the desktop, RAM memory and hard drive by referring to desks, work tables and filing cabinets.