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 Am/G
Am/G is Am with G as the lowest note. Slash chords like this let the bass line move step by step between chords.
Chords in the key of A minor
These chords share notes with Am/G and sound natural next to it.