There are no less than 92 versions of the ballad "Barbara Allen." The one you ask about, Bonny Barbara Allan is considered to be the oldest and probably closest to the original. Ballads weren't written down and as they were passed from story teller to story teller they often changed. This Scottish ballad was about a young man named Sir John Graeme. The setting is in a village during the Martinmas time. This is generally thought to be around the feast of St. Martins and it is November 11. It was a time of feast and celebration.
Sir John sends his men to go find Barbara Allen. They find her and tell her that Sir John wants to see her. She makes her way to where he lies ill. She has taken her time, the ballad states: "O hooly, hooly rose she up, To the place where he was lying." Hooly means slowly. When she pulls back a drape to see him she sees that he is very sick and he tells her he is sick in love with her. She rebuffs Sir John because he had offended her in a pub during an earlier time. He is so heartbroken he dies of grief and unrequited love.
Newer versions of the ballad have her following him in death due to her bad choice of not accepting his love and guilt. They are reunited in death, but that is not in the original.