Detach refers to the act of separating an Xyz Material attached to an Xyz Monster either as a cost or by an effect (that uses this term), sending it to the Graveyard as a result.

When a Xyz Material is detached, it is not treated as it leaving the field, since Xyz Materials are not treated as being on the field; so the effects of monsters like "Sangan" and the conditions of monsters like "Plaguespreader Zombie" are not applied.