x/y is VPIP (voluntarily puts money in pot %) and PFR (preflop raise %).
It's used in tracking software to get an idea of how loose or tight someone is. A 12/10 to 17/15 is a TAG. A 20/18 to 27/24 is a LAG. A 38/6 to 44/8 is a fish. Just as an example.