The D major seventh flat sixth chord is a 4-note chord consisting of the notes D, F#, Bb and C#.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as DM7b6.
Chords that are a subset of DM7b6. These contain less notes but all of them are included in DM7b6.
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 DM7b6 chord. Feel free to save or share the image as needed.