The Revoke Title button in the Diplomacy pane allows you to take titles away from your direct vassals, adding them to your own, after which you may do with them as you please. This button will only appear when conducting diplomacy with a direct vassal.
Clicking the button will first bring up a list of the titles which may be revoked.
Selecting the title you wish to revoke will then bring up the warning screen. This screen has two variants, depending on whether the target of your revocation has done anything to deserve it. If not you will see this.
If your subject hasn’t done anything wrong, you will be warned of the various penalties for your action; a 100 point Prestige penalty, a -80 Revoked my Title opinion penalty with the individual that had owned the title, and a (stacking) -20 Revoked Vassal Titles penalty with your remaining vassals.
If the title holder has done something rebellious, like declaring independence, he or she may have one of his or her titles revoked without suffering realm penalties. (The title holder will still be pissed.)