Basically, she was a woman who had seven demons cast out of her by Jesus and so she became a follower of Jesus. She was present during Jesus’ crucifixion, present at his burial, and she was the woman who told people that Jesus came back and the tomb was empty. She was the first person to see the risen Jesus, and that was a pretty big deal overall.
There’s a lot of confusion around her because there’s a few other Marys in the Gospels. She’s mainly confused with the Mary of Bethany who washed Jesus’ feet with perfume and her own hair before the crucifixion, and that confusion gets her confused with an unnamed woman who also washed Jesus’ feet with her tears, hair, and perfume. Someone more versed on the history could probably explain how the myth of her being a prostitute got started, but that is totally false and has not a thing to do with scripture.