Okay kiddo, imagine you're drawing a picture. You might start by drawing a circle for the head, then two dots for the eyes, and a smiley mouth. These are little parts that make up the big picture.
In music, a motif is like those little parts. It's a short musical idea that gets repeated throughout a song or piece of music to help tie things together.
For example, do you know the song "Twinkle, Twinkle, Little Star"? The first four notes are a motif: "twinkle, twinkle, little star. . ." Those same four notes are repeated over and over again throughout the song, helping to make it catchier and easier to remember.
So, just like in a picture, a motif in music is a small part that helps make the whole thing come together.