I should let Scott or boo or any of the more formally trained musicians answer this, but the II refers to the second chord of the scale (key) you are playing in. In boo's example, playing in the key of 'C', the second chord is 'D', but in this case it is "bII" so the 'D' is flatted ... (is there such a word ) ...

As I said, I am not as formally trained and don't claim to have a great ear, but I have used that chord sequence for years because 'it sounded good' ... It's nice to know that I was right ...
t.