Once a wedding date has been set the banns of marriage, commonly known simply as "the banns" (from an Old English word meaning "to summon") are announced. This is a notice, usually placed in the local parish church or registery office, which tells everyone that a marriage is going to take place between two people.
The purpose of banns is to enable anyone to raise any legal problems as to why the two people shouldn't get married, basically it is to prevent marriages that are legally invalid. Impediments vary between legal jurisdictions, but would normally include a pre-existing marriage (having been neither dissolved nor annulled), a vow of celibacy, lack of consent, or the couple's being related within the prohibited degrees of kinship.