I'ts short for "Computer Generated", but in VNs it's always used as a shorthand for those event scene illustrations you see every now and then. The demo should have had only one, but there are more in the completed story.
I don't really know what you mean by "event scene illustration" either. ^^" Could you explain further?
Generally speaking, visual novels are told through what looks like a puppet show, with sprites facing the reader or each other and a text box with their dialogue.
Particularly important scenes/events get a special illustration that's drawn specifically for that scene. I seem to remember that the demo here should have a scene with Marco and Ayumu on stage: that's the event scene in this particular demo. There are a few more in the complete game which were drawn already.