How can I enable this effect on the desktop?
Many thanks.

Assuming you have desktop effects enabled go to system settings -> desktop -> desktop effects (it may be selected automatically, you can enable desktop effects here) -> window management, then check "desktop grid" if it isn't already. Click the wrench to the right, then check "Use present windows effect to layout the windows" if it isn't already. Then on the right click "show desktop grid" and either remember the shortcut there or make your own. Then click "ok" and "apply" if you need to.

Many thanks.
It was already enabled.I just didn't know what it's called and the key combination to bring it up.
Many thanks again.

P.S. Is there a manual of list of some kind with the shortcuts to such
desktop effects?

Go to system settings -> keyboard and mouse -> global keyboard shortcuts. Change the drop-down at the top to "kwin" and it will show you all of the available shortcuts, including the ones currently assigned. If you click the heading of the "global" column in the table (perhaps twice) it will group all of the existing shortcuts so you can see them more easily.