Purpose Of Charge Indicator Light

The "ALT" indicator light is a warning, when that's on, the battery is draining, you are getting no charge.

Correct. The light should come on when your first turn the key ON as a bulb test, but should go out within a second or two of the engine starting. If the light flickers on at stoplights it could just mean your belt is loose or idle speed is too slow. If it comes on dimly it's usually a sign of a bad diode in the rectifier bridge. If it comes on brightly and stays on it means the alternator is not charging and you'll eventually drain the battery.