The D flat major seventh chord is a 4-note chord consisting of the notes Db, F, Ab and C.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as Dbmaj7.
Chords that are a superset of Dbmaj7. The chords include more notes but always Db, F, Ab and C.
Chords that are a subset of Dbmaj7. These contain less notes but all of them are included in Dbmaj7.
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 Dbmaj7 chord. Feel free to save or share the image as needed.