The term “tonic” refers to the first note of a scale, and also to the chord that is built upon that note. In A major, A is the tonic note, and the chord built on it (A-C#-E) is the tonic chord. It’s an important note and chord because it so happens that they usually appear more often in choruses than they do in verses. The tonic function is analogous to “home.” If you took a walk around your city, finally coming home denotes a feeling of rest and completion. And that’s the sort of function it has in music.
For obvious reasons, you can easily think of songs as being a bit of a musical journey. So that concept of the tonic chord as a resting point, particularly when accompanying the tonic note, becomes an important focus.
As you know, verse harmonies will accommodate more “fragile” progressions than choruses. A fragile progression is one which is slightly more ambiguous in tonal direction. For example, a progression such as ii-iii-vi avoids the two chords that strongly indicate key: the tonic chord and the dominant (V) chord.
Because it’s a tiny bit ambiguous, it will work well in verses where you’re setting up a situation, or describing a story. But choruses are different.
A chorus melody, and its accompanying harmonies, will accept the tonic note and chord more easily. Why? Because chorus lyrics tend to be more “conclusive”, and offer a resolution to the verse situations. Because of this, you’ll want accompanying harmonies to similarly be more conclusive, and tonally stronger. The presence of the tonic note will be an even more obvious strengthening of the tonic function.
Having said that, verses can also accept strong progressions, but the verse melody that goes along with a strong progression will usually avoid the tonic note in important positions. (i.e., not often at the start of phrases).
Think of Michael Jackson’s “Billie Jean” as a good example of this concept. The song is in G minor (actually adjusted a bit flat), and though the tonic chord plays a vital role in the verse harmonies, the tonic note is used sparingly: just at the ends of phrases, and in a low range.
The chorus of Billie Jean” features the tonic note much more, and this use, particularly in the upper register and at the beginnings of phrases, adds a sense of structure and musical focus that adds momentum and direction. It’s a perfect song for demonstrating song energy. It barely gives us the tonic note in the verse, then gives it much more in the pre-chorus (but with less focus on the tonic chord). Then the chorus is the pinnacle, when tonic note and chord finally come together in a musically satisfying way.
So don’t let your melodic and harmonic construction be “an accident”. Follow the structural plan that makes many hit songs the hits they are: give the tonic note and chord most prominence in the chorus of your song.