Hello! So, Markan priority is a theory that says the Gospel written by a man named Mark was actually the first one written out of the four Gospels in the Bible.
So, let's first talk about what the Gospels are. The Gospels are four books in the Bible (Matthew, Mark, Luke, and John) that tell the life story of Jesus. Each book is written by a different person, but they all talk about the same things.
Now, back to Markan priority. The idea is that when the people were collecting stories about Jesus to write down, Mark wrote his book first. Then, the other writers used Mark's book to help them write their own versions of the story.
Think of it like building a tower out of blocks. Mark built the first tower with his blocks, and then the other writers used his tower as a guide to build their own towers, but they might have added different blocks or changed things around.
Some people don't agree with this theory and think that the other writers could have had their own sources or just wrote their own stories without using Mark's book. But Markan priority is still an interesting idea to think about when we're trying to understand the history of the Bible.