D/F♯ means “D with F♯ in the bass”: play a D major chord and make F♯ the lowest note. Because F♯ is already in the chord (its major third), this is the first inversion of D.
The notes, low to high, are F♯, D, A. The library above opens on D/F♯ with 14 shapes that keep F♯ at the bottom; tap one to hear it.