Yeah I know.. just have to get used to it. It's basically a measure of how far above or below you are from average. You don't get that with ranks for example because you don't know that the difference between rank 15 and 16 is the same as between ranks 2 and 3, and you don't get that with just saying how many passer rating points you are above the mean. So the z-score is a "normalized" measure of how far above or below the mean you are. You get that by dividing the actual number of passer rating points above or below the mean by the standard deviation. Anyway, it's really useful because you can directly compare across eras. In each case you're just asking how far above or below league average each QB was. Much better than quoting raw passer rating which has increased for everyone over time because of changes in rules or how they're interpreted.