Plainsong is like a very old kind of music that was sung by people a long time ago in places like churches and monasteries. It was made up of simple, beautiful melodies that were passed down through the years without being written down on paper.
A Medieval Music Society is a group of people who really love this kind of plainsong music and other music that was popular during the medieval times. They get together to sing this music and talk about it with each other, kind of like a big club. They might perform this music for other people at concerts or shows, or teach others how to sing it too.
People who really love plainsong and other medieval music might also study it in school or do research to learn more about it. They think it's really important to keep this old music alive so we can all enjoy it and remember the past.