It is the ratio that the rocker arm opens the valve relative to the cam.
Lets just say you have a cam that is .325" lift at the cam lobe and you are using 1.5 ratio rocker arms, that means the rocker arm makes the cam lobe lift 1.5 times greater at the valve for a valve lift of .487", the same cam with 1.6 ratio rocker arms would have a valve lift of .520"