A C♯ Major 6 (C♯6) chord contains the notes C#, G#, F, A#. It is built from the major 6th formula 1 - 3 - 5 - 6, with C# as the root.
Is C♯6 the same chord as D♭6?
Yes — identical frets, identical pitches. Which name you use depends on the key you're in: sharp keys like E or B call it C♯6, while flat keys like A♭ or D♭ call it D♭6. Writing it the way the key signature does keeps the notation readable, but there is nothing to relearn on the fretboard.