Numbers on the dots are fingers: 1 index, 2 middle, 3 ring, 4 pinky. ○ means play the string open, × means don't play it. Tab reads from the low E string to the high E string.
About G/F♯
G/F♯ is G with F♯ as the lowest note. Slash chords like this let the bass line move step by step between chords.
Chords in the key of G major
These chords share notes with G/F♯ and sound natural next to it.