The basics of the "Tonic" then...
The tonic is the first note of the scale.
It gives the scale it's name.
It will be the center note of the composition.
It's so important, that if the composition doesn't end with this very note, it will sound "unfinished".
In Cmajor, this note is C.
From this note, you also create the Cmajor chord by adding the third and the fifth.
C E G
This chord is called "the tonic chord".
It's not enough that the composition ends with the tonic chord, it has to end with the note C as the top note. If you play the tonic chord (from bass to treble) C E G, it will sound as if the composition ends with the tonic chord, but with the G as the main melody note, because G is then highest up. This isn't good enough. You have to change the order of the notes so that you play (from bass to treble) E G C. This is the same chord, Cmajor, but now with C as the top note, which we hear as "the melody".
It's equally important that C is the note lowest in the bass, otherwise it will also sound "unfinished". Therefore, the left hand should add C in the bass.
Normally the composition also starts with the tonic chord, but at this point the note C isn't so important. That is because the composition will deal with a lot of "tensions" between the different notes of the scale anyway, so how it starts is less important than how it finishes.
Next, all the rest of the notes of the scale, that is numbers 2-7, in Cmajor:
D E F G A B
will all have a special relationship to the tonic note C.
To investigate these relationships, we need to do a few musical experiments:
Play C four times, followed by G four times.
What happens?
After that last G, you will hear an enormous tension.
If you are forced to play only one note after that, and then the little piece must end, which note must you play?
Now play C four times, followed by B four times.
What happens? How must you finish this?
Now C four times, followed by D four times.
How must you finish this?
As you can hear, there are some obvious forces going on. These have of course been recognised for ages. I'm sure you'll agree that the most powerful of these three experiments was the one using C four times followed by G four times. For this reason, the G (or the fifth step in the scale) has been called "the Dominant".
This is very important to learn and understand. The Dominant will be prominent in almost every pop- and rock song ever written. It is also very prominent in the music of Mozart and Beethoven. A little bit less with Chopin, who lived a bit later, and by the early 20th century it was avoided all together by composers like Arnold Schoenberg.
The other two notes have come to be called other things, but they are not so important to memorize: the D (or the second step of the scale) has come to be called "the Supertonic" (meaning "the note above the tonic"), and the B (or the seventh step of the scale) has come to be called "the Leading note" (because it kind of leads so smoothly back to the tonic).
As you have seen, three notes in the scale, the 2nd, 5th and 7th, all pull, or "break down", as you put it, to the tonic note, the 1st.
As it happens, these three notes also form a perfect Dominant chord, in this case a perfect Gmajor chord:
G B D.
Crystal clear?:-) I'm sure the topic will be moved again soon, to the "theory board:)