Being attached is another way of saying that an Xyz Material(s) is being placed underneath an Xyz Monster after it was successfully Xyz Summoned. An example of this is the effect of "Number 101: Silent Honor ARK".
Monsters are usually the type of card that is attached to Xyz Monsters, but some cards allow the attachment of Spell or Trap Cards, such as "Overlay Regen" and "Xyz Reborn".