Well kiddo, have you heard of the holy book called the Bible? Yes? Good. Now, the Bible has four important books in it, called Gospels. Matthew, Mark, Luke, and John are the names of these books. These books tell us about the life, teachings, miracles, and story of Jesus Christ.
Now, a Gospel harmony is like a beautiful music that is made by combining different voices into one song. Similarly, Gospel harmony is a way of combining the four Gospels and creating one single story about Jesus.
Imagine you have four friends and they all saw something very interesting. One friend might tell the story differently from another because they were standing in a different place, or they might have focused on different things. In the same way, each Gospel writer wrote their own book about Jesus, and they may have included different details or stories that the other writers didn't include.
But with a Gospel harmony, we can combine all four books and get the full picture of Jesus' life and teachings. We can see what they all said and find similarities and differences. This is helpful because we can get a bigger and better understanding of what Jesus did and said during his time on Earth. Cool, right?