This story involves the same Mary and Martha we learned about in the Jesus Raises Lazarus from the Dead lesson. We learned in that story that Mary and Martha were good friends of Jesus, along with their brother Lazarus. He would stay at their house frequently as he was traveling around Jerusalem.
In this story, Mary and Martha were entertaining Jesus at their house. Martha was very busy serving Jesus, preparing the meal, cleaning, etc. Mary was sitting at Jesus' feet, listening to Jesus teach. We don't know if Jesus was just teaching Mary, or whether others were there.
But Martha became indignant at Mary that she wasn't helping out. You can imagine in your head Martha passing by the doorway to the living room on her way to get food supplies, taking the trash out, or taking the food to the dining room, each time getting more and more frustrated that she was doing all this work herself while her sister Mary was just sitting down in the living room with Jesus. Martha finally snapped: "Lord, my sister has left me to do the work by myself. Don’t you care? Tell her to help me!" (Luke 10:40).
But Jesus had a very important lesson for Martha. He told her that Mary had a choice. She could have gotten very busy with all the serving, preparing, and cleaning - just like Martha. Or she could have chosen to spend time with Jesus and learn from him. Jesus told Martha that Mary had made a good choice.