Okay kiddo, so you know that music has different notes, right? Like "do, re, mi, fa, sol, la, ti, do" (sings). Well, when we talk about a subtonic, we're talking about the seventh note in the musical scale.
For example, if we're playing music in the key of C (which means we're using the notes "C, D, E, F, G, A, B"), the subtonic would be "B."
Now, to make things a little more complicated, the subtonic is also sometimes called the "leading tone." This is because it usually leads to the next note (which would be the "tonic") and kind of sets up the next part of the song or melody.
So, when we say "subtonic," we're talking about the seventh note in the scale and how it helps guide the music to the next part. Does that make sense, kiddo?