The tonic note is the note that represents the key your song is in. So if you’re writing in G major, the tonic note is G. The tonic note is of tremendous value in strengthening your song’s overall structure; its importance cannot be overlooked. How you use it is what Songwriting Principle No. 8 is all about:
The presence of the TONIC NOTE will strengthen the underlying structure of a melody. Choruses can and should feature the tonic note in its melody more than verses.
It’s very difficult to make generalizations about writing songs, mainly because songwriting is an art form. Art sometimes defies explanations as to why it works. The composer Ravel wrote the orchestral work “Boléro,” and it consists of one 32-bar melody repeated eighteen times. If we ever suggested that approach as a viable compositional technique, the results would probably be disastrous. But for Boléro, it works. In typical songwriting, the presence of the tonic note in the chorus will strengthen your song’s overall structure. There are probably songs that completely avoid the tonic note, but to make a safe generalization, using it (especially in choruses) gives the lyric of a chorus more strength.
How does it work? If you think of the analogy of taking a journey, the verse is the songwriter traveling away from home, and the chorus is returning home. In other words, the verse is the story, and the chorus is a summation of our feelings and emotions once the main part of the story is concluded. The tonic note (and accompanying tonic chord) is the best musical analogy we have for the concept of “home”. Featuring it, particularly in choruses, provides a “home base” from which the songwriter can make conclusive, emotion-based observations about the subject of the song, which is what choruses need to do.
-Gary Ewer, from “The Essential Secrets of Songwriting” website