That usually means to play a D chord with an F# in the bass. That's harmonically okay, because an F# is a note in the D chord (D F# A).

That sort of notation (chord-symbol, slash, note) is often used to indicate things like a "walking" bass, which is sort of like a bass-run. For example,

D/D - D/C# - D/B - D/A (or A/A).

Here, you would play a D bass (4th string open), strum or pick the top three strings, play a C# (5th string, 4th fret [fret it with the pinky]), strum or pick, play a B (5th string, 2nd fret [fret it with the 1st finger]), strum or pick, then play an open 5th, and strum or pick.

This sort of thing is a lot easier to show that to explain.

Don Firth