There are, you see, certain things that cannot get inside one's house unless one invites them." Dracula, as a vampire, can only enter houses when invited, whereas Santa can go in and out of any house he wants.
Its applications to different types of supernatural creatures have expanded over the years. Traditionally, this trope applied to mythical creatures in general, and there's a load of things that are supposed to invite them in, like putting new shoes on the table, or bringing white hawthorn into the house. Once invited, don't expect locked doors to be an obstacle to their entry. Often such beings will try to gain entrance by tricking a person into believing they are someone else. Going back in various mythologies is the idea that supernatural beings - such as vampires, ghosts, demons, witches, what have you - cannot enter into a home unless invited to do so.