Tonic:
The tonic is the most stable note in a scale, and it serves as the tonal center. It is the note that gives the key its name, and it is the note that all other notes in the scale relate to. The tonic is typically the first note of the scale and is often used to begin and end a musical phrase or section.
Dominant:
The dominant is the fifth note of the scale, and it is the second most important note after the tonic. It creates a strong sense of tension and resolution when played in relation to the tonic. The dominant is often used in cadences, which are musical phrases that signal the end of a section or piece.
When referring to letters as tonic or dominant, it means that the note represented by that letter is functioning as the tonic or dominant within a particular scale or key. For example, in the key of C major, the note "C" is the tonic, and the note "G" is the dominant.
Understanding the functions of tonic and dominant notes is essential for music theory and analysis. They provide the foundation for tonal harmony, melody, and chord progressions, and they play a crucial role in creating a sense of musical structure and tension.