Okay kiddo, so imagine you have 6 friends and you want to figure out who your middle friend is. That middle friend would be your mediant!
Now let me explain it using music. When you listen to a song, you might notice that some notes sound good together and some don't. When we talk about mediant in music, we're talking about two specific notes that sound good together.
Let's say you're playing a song on the piano and the first note you play is C. The mediant would be the note that sounds good with C and is two notes up or down from it. So in this case, the mediant would be E.
So next time you listen to music or play an instrument, try to listen for the mediant and see if you can hear which notes sound good together!