Question: What's a tonic?
Albert's reply: The tonic is the first note of any scale. If the key is C major, then C is the tonic. If you're in A-flat major, then A-flat is the tonic.
Since the tonic is the main note in any key, it's sometimes also called the key note. (Hence the pun: key-notes!)
The tonic in music can also refer to the chord (triad) built on the first scale degree. In C major, the C major triad (C-E-G) is the tonic triad or chord. In A-flat major, the A-flat major triad (A♭-C-E♭) is the tonic triad.