The C major seventh flat sixth chord is a 4-note chord consisting of the notes C, E, Ab and B.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as CM7b6.
Chords that are a subset of CM7b6. These contain less notes but all of them are included in CM7b6.
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 CM7b6 chord. Feel free to save or share the image as needed.