A janitor is someone that cleans and maintains buildings. It's common to see a janitor sweeping a floor. When someone is "swept off their feet" it means that they are suddenly very attracted to someone in a romantic way. The text is tying in the job duties of a janitor with their daughter being romantically attracted to someone using the word "swept". 
She was only a Janitor ' s daughter , and she was often swept off her feet .