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.
Interactive piano diagram for the D major seventh flat sixth chord
Chords that are a subset of DM7b6. These contain less notes but all of them are included in DM7b6.