Install Windows Management control or wmctrl:
sudo apt install wmctrl
now, typing the following command in the terminal will show the desktop:
wmctrl -k on
That is OK, but it is easier with a keyboard shortcut.
Open System Settings->Keyboard (or search keyboard in Applications for Loki 0.4 Elementary OS and possibly other distributions)
Go to 'Shortcuts tab
Go to 'Custom'
Click the '+' and type
wmctrl -k on and 'Enter'
The shortcut will show is 'Disabled' on the right
Click on 'Disabled' and click on the shortcut you like for example the 'Windows' button + D, also known as 'Super + D'
Try it: click you shortcut. You should see the desktop
That is it. You are done.
Note: this answer is basically the same as Alex's above, I just added more detailed steps (and tried to +1 his, but not enough reputation :)