Ternary in music?

"Ternary" simply means that the piece of music you are talking about happens in three distinct parts. For example, it starts with part "A," then goes to part "B," then maybe back to part "A" again.

Mozart has a good example:
Part A- "twinkle twinkle little star, how I wonder what you are"
Part B- "up above the world so high, like a diamond in the sky"
Part A- "twinkle twinkle little star, how I wonder what you are."

It can, of course get MUCH more complex than that, but the important thing is that there are three obvious parts. Doesn't matter if one of the parts is repeated (like Twinkle pt A above), as long as the other, different part is sandwiched in the middle to divide the repetition.