Okay kiddo, Marcan priority is like when your friend tells you a story and then another friend tells you the same story but with a few extra details. It means that the person who told the story first has priority or came before the other person who added more information.
In the Bible, there are three books called Matthew, Mark, and Luke that have very similar stories about Jesus' life. Marcan priority means that out of the three books, Mark was written first and Matthew and Luke used Mark's story as a guide when they wrote their own books. It's like they used Mark's story as a foundation and then built upon it with their own details.
So remember, Marcan priority means that Mark's story was written first and then Matthew and Luke used it as a guide for their own versions. It's kind of like when you use a coloring book's picture as a guide to color your own picture, but you add your own special touches to make it unique.