“Faillad bannock” in Scottish means basically “leftover bannock.”
If you left an uneaten bannock in the house overnight, that would grant admission to the Faeires into your house.
You could prevent Faeires admission by breaking a piece off of the Fallaid Bannock, by putting a coal on top of it, or by making the bannock with a hole in the centre of it. The traditional way to make a hole was to put the thumb of your right hand in the centre, and turn it clockwise.