This was the woman who was with Jesus and the apostles. She comes up in the early part of the Catechism during the Profession of Faith. She tended Jesus's body after the crucifixion. As such she was one of the first to discover that his body was missing because he had risen from the dead.
Her reputation remains tainted because some believe that she was a prostitute prior to meeting Jesus and the other apostles. Like some of the men, she abandoned her work to follow Jesus. Hers was simply yet another case where Lord Jesus defied social conventions and stigmas and welcomed her genuine belief and interest in his teachings instead of rejecting her because of a bad reputation in society.