Mary Boleyn was a lady who lived a long time ago in England, when there was a king named Henry VIII. She was the sister of another lady named Anne Boleyn, who later became very famous because she was married to the king. Mary was also married and had children, but she was not as famous as her sister.
Mary and Anne both served as ladies-in-waiting to the queen of England, which means they helped her with her duties. One day, the king noticed Mary and started to have a romantic relationship with her, even though she was already married. This was not very nice, because it was against the rules and could cause a lot of trouble.
Eventually, the king lost interest in Mary and started to pay attention to her sister Anne instead. Anne also became involved with the king, but she was determined to become his wife. They had a big fight with the church because the king was already married, but eventually they were able to get divorced and marry each other.
Mary Boleyn was still alive when her sister became queen, but she was not treated very well by her family and was not invited to important events. She ended up living a quiet life with her husband and children, and she is mostly remembered for being the sister of Anne Boleyn and the king's former mistress.