The C sixth chord is a 4-note chord consisting of the notes C, E, G and A.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as C6.
Chords that are a superset of C6. The chords include more notes but always C, E, G and A.
Chords that are a subset of C6. These contain less notes but all of them are included in C6.
In case you prefer a non-interactive variant of the chord chart, we've embedded a PNG image below that shows the notes for the the C6 chord. Feel free to save or share the image as needed.