D with an F# bass.
This is known as an inversion of D. There is already a F# note in the chord on the first string. So this is simply changing the order of the notes in the chord.
Some people like to finger all notes but I think the thumb works just fine. Now you can strum six strings on this chord.
However, D/F# is best played with the bass note picked by itself. Try playing B D DuD. (Bass, Down, Down Up Down. 1 2 3+4) Try it in the chord progression G, D/F#, Em and C. Play a bar of each.
Listen for the run in Come Monday by Jimmy Buffett,
G D/F# Em D
I spent four lonely days in a brown LA haze
Probably has a capo on the second fret.