hi Markem
the Em/B means play an E minor chord and play B note as the bass.
for Am7/G, it means play an Am7 chord with G note as the bass.
this is sometime used in songs where the arranger wants to have a Bassline that is running in a linear fashion. For example a song in C major key. The chords are C, Dm, Em, F, G7, Am, Bm7-5. He could play C with C bass, Em with B bass, Am with A bass, Am7 with G bass, F with F bass, Em with E bass, Dm with D bass and G7 with G bass.
See how the bass line goes? C, B, A, G, F, E, D, G. From C to D, it is moving in a decending order.