G/F♯ means “G with F♯ in the bass”: play a G major chord and make F♯ the lowest note. F♯ is not one of the chord’s own notes, so the bass adds colour as well as moving the bass line.
The notes, low to high, are F♯, G, B, D. The library above opens on G/F♯ with 14 shapes that keep F♯ at the bottom; tap one to hear it.