Shortcuts · Desktops and spaces
Show the desktop: the Mac and Windows shortcut
macOS
F11
Windows
Win+D
What catches people out
On a Mac laptop that's fn+F11 unless you've set the function row to behave as F-keys. ⌘F3 does the same on keyboards with a Mission Control key.
See it in the full table alongside 69 others, or search for the one you actually came for.