A “mail-order bride” is a person, usually a woman, who decides to find a spouse via an agency which puts them in a catalogue of brides.
They are contacted by possible spouses interested in them and can decide to marry them.
Agencies that provide this service are paid. Mail-order brides usually cover the cost of their advertisement on the catalogue, while spouses usually cover the rest of the costs (legal fees, travel expenses for the brides’ relocation, bonus for the agency, etc.)