Betrothed means "formally engaged." If you are betrothed, then, congratulations! As they say in the song, you’re going to the chapel — you’re going to get married.
Betrothed came into English through a combination of bi-, or "thoroughly," and treowðe, the Old English word for "truth, a pledge." If you are betrothed, you are completely and formally pledged to someone. Betrothed has a slightly more old fashioned and formal feel than its synonym engaged, but both describe people who are between "Will you marry me?" and "I do."