Ubuntu will dim your screen when you are inactive, even if your computer is plugged into AC power. If you don't want it to do the screen, you can change this setting. Go to System > Preferences > Power Management. You'll then see tabs for On AC Power and On Battery Power, pick the setting you want to adjust. In the Display section, you can slide the slider to adjust the amount of time of inactivity before it dims the screen. If you slide this slider all the way to the right, it'll change from being a time to be Never.