Okay kiddo, so you know how we have four books in the New Testament of the Bible that tell us about the life of Jesus? They are called Matthew, Mark, Luke, and John.
Well, some people think that maybe Matthew, Mark, and Luke are very similar because they used each other's stories to write their own. This is called the "synoptic problem."
The Griesbach Hypothesis is a theory about which book came first out of the three similar ones (Matthew, Mark, and Luke). Some people think that Matthew wrote his book first and Mark used some of Matthew's stories in his book. Then, Luke used both Matthew and Mark's stories when writing his book.
But some other people think that Mark wrote his book first and both Matthew and Luke used his stories when writing their own.
It's still a mystery which is correct, but lots of smart people have studied it for a long time and they all have different opinions.