The tonic is the first scale degree of a diatonic scale (for example, the major scale, is a diatonic scale). Sometimes it’s called the 1st scale step, or the 1st. The tonic is often confused with the root, which is the reference note of a chord, rather than that of the scale.
It is often represented with the Roman numeral I.
Scales are named after the tonic. For example, the C scale’s tonic is a C note.
The triad formed from the tonic note is called the tonic chord (or may just be refer to it as’ the tonic’). It’s the most important chord in a song, because the tonic is the tonal center where all other tones resolve to. Therefore most songs begin and end with the tonic. For example, a song in the key of C would have C as it’s tonic, use the C major scale, use the C major chord as it’s tonic chord, and most likely begin and end on C. So, in the context of a song, if we’re in the key of C. C is the tonic or root note.
The tonic is a pretty simple concept to learn once you realize it’s not some sort of herbal remedy. It’s a very important concept to learn, because you’ll hear it talked about a lot.