A titer is a measurment of how much of something is there, or a "quantitative" test as we might say in a chemistry lab. They perform dilutions and see how many dilutions they can go through and still detect the antibody or whatever it is they are testing. Thus a titer result is displayed as 1:160, 1:640, or whatever... meaning they can find it in as small a dilution as one part in a 160 dilution (or whatever the second number is).
So an IgG titer is a measurment of how much IgG antibody they found.