The Mind Stone is used to store experience levels (XP). It rapidly consumes your experience levels when activated by pressing (Toggle Key) to be obtained again at any time by holding the stone and pressing (Toggle Key) then right clicking.
Whenever you right click it gives you a level back, up to the maximum possible amount stored. Useful for preserving your precious Experience.
Currently there is a bug which does not return the levels when you right click the Mind Stone. This is a bug and can be fixed by either disconnecting and rejoining, or switching between End, Nether, and Normal worlds, the result being your levels being returned.
There is a bug that will grant infinite levels as long as you continue to log right click and log off without reactivating the stone between relogs. It grants 1 level per right click for as many times as you right click. I would suggest disabling this on your servers if you care about people getting easy silk touch. beyond that there is really no point in having infinite levels considering that all of the power items make enchants obsolete.