The F sixth chord is a 4-note chord consisting of the notes F, A, C and D.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as F6.
Chords that are a superset of F6. The chords include more notes but always F, A, C and D.
Chords that are a subset of F6. These contain less notes but all of them are included in F6.
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 F6 chord. Feel free to save or share the image as needed.