So, York Mystery Plays are like a big play with a lot of parts. It's like a Disney World ride called "It's a Small World" but instead of seeing different countries, you see people acting out stories from the Bible.
Just like in a big play, there are different characters who act out different parts of the story. Some people are dressed up as animals or angels or even the devil! They have lines to say and songs to sing.
The York Mystery Plays are really special because they have been going on for hundreds and hundreds of years. People in York started doing them back in the Middle Ages, which was a really long time ago.
Back then, not many people could read or write so they learned about the stories from the Bible by watching plays like this. It was a fun way to teach people about religion and the stories from the Bible.
Nowadays, people still love the York Mystery Plays because they are a part of history and tradition. The plays are only performed every few years, but when they are, people come from all over the world to see them. It's like a big party where everyone gets to watch a really fun, old play.