"Forge ahead" means to make good progress. The word "forge" is a verb that references shaping a metal object by heating it in a fire or furnace. A blacksmith is a person who makes and repairs, or forges objects from iron over a fire. The joke is playing on the word "forge" and its different meanings, and relating it to a blacksmith's daughter.
She was only a Blacksmith ' s daughter , but she knew how to forge ahead .