WebUVs keeps your work in your browser as you go, so a reload, or closing the tab, doesn't lose it.
Open WebUVs again in the same browser and it puts your work back where it was, saying "Restoring your previous session" while it does.
Only in this browser, on this computer: nothing is uploaded. So your work doesn't follow you to another browser or device, and a private window keeps nothing once it's closed. Clearing the site's data in your browser's settings clears it too.
Opening another model, or a shape or example from the File menu, replaces what's kept. To keep your work, save it first.