Attuneable

An attuneable item must be attuned before it can be equipped. This is done by right-clicking the item and choosing Attune from the context menu, or alternately, equipping the item and then clicking Yes when a dialog asks whether you want to attune the item.

Doing so means that the item is bound to the player, and can not be used by anyone else. (It can still be sold to NPC merchants.)