C/G means one row is in C and the other is in G.

On an Anglo concertina, each row of buttons is a bit like a harmonica, with push and pull on the bellows corresponding to blow and suck respectively, playing a scale in one key per row.

That's for two rows - if it had a third row, that would contain an assortment of accidentals and reverse-direction notes to make up for what's missing from the other two.