Terribly sorry. The threshold...at the bare minimum, to cross the threshold means to enter into a building. In a much larger sense, it's symbolic of the ownership a person has over a building. If the proper ownership is there, the threshold can be warded in order to keep certain people, species, or items out of the building.

Truthfully, if you'd have asked me about it before Portland, I'd have said it was utter nonsense, but threshold magic worked there, so I can only assume it would work elsewhere.