Whats a cue number?

Well last night we had a showing of our festival piece Wrong for Each Other and lets just say that the dirctor should be very happy that I was on the board at least for lighting as I didnt have any cue numbers to speak of. I went on go or on my own.
I had the levels all set and the faders set as groups to make cross fades (bringing up lights and takeing others down) but the dirctor didnt want cross fades they wanted a brown out between scenes. Lets hope that it goes better in Grand Falls.