What does Ternary mean?

The proper terminology is Ternary Form or Rondo forum.

What this means is "three sections", as in a A B A form of composing. The piece begins and ends with the section A (original theme), then section B is something different, then the piece returns to section A, possibly with some modifications.

An overly simplified illustration of this might be:

* Section A: tra-la-la-be-do * Section B: fa-so-so-le-to * Section A tra-la-la-be-do-dum You see, we have started out with a basic theme, then went into the B section and returned to A, but added something special at the end.