To attach is place an Xyz Material underneath an Xyz Monster. Attached monsters are not considered on the field, so effects like "Ceasefire" or "Compulsory Evacuation Device" do not affect them. Also, they cannot be Tributed for a Tribute Summon or used as material for a Fusion Summon or Synchro Summon. Since Tokens can only exist on the field, they cannot be attached to an Xyz Monster.
Most of the Xyz Monsters' effects are activated by detaching the material monster(s) attached to them. A monster can be re-attached to an Xyz Monster after it was detached though only with the anime effect of "Number 19: Freezadon". Also, monsters can also be attached to an Xyz Monster by methods other than the Xyz Summon, as shown with "Overlay Regen". Trap Cards, besides Trap Monsters, can also be attached, as shown with "Xyz Reborn".