The default keyboard shortcut key for showing the desktop in Ubuntu is ALT+ALT+D, but you can also change it to a custom key if you want to.
How To Change Ubuntu Show Desktop Keyboard Shortcut Key
1. Goto System > Keyboard Shortcuts
2. Look for “Hide all normal windows and set focus to the desktop” (for windows users, that means show desktop!) and there should be a default shortcut key set to CTRL+ALT+D.
3. Enter a new shortcut key. Windows users will tend to use the infamous “windows key” + D combination, which will translate to Mod4+D in Ubuntu.
That’s it. Pressing “Windows Key” + D will show you Ubuntu’s desktop in full MS Windows fashion.