Okay kiddo, have you ever heard a story from your grandma or your teacher or your favorite book that has the same idea or theme as another story you've heard before? That's what a motif is. It's like a little idea or a pattern that shows up in different stories from different cultures and different times.
A motif-index is like a really big list that tells us all the different motifs that show up in different folk stories from all over the world. It's like a big library of ideas that we can use to study and compare different stories.
So, if we want to learn about all the different stories that have a prince in them, we can look up the "Prince" motif in the motif-index, and it will show us all the different stories from all the different cultures where a prince appears. It's like a big treasure map that helps us find all the hidden gems in the stories we love.