What does it mean when my car's red battery light comes on?

First check the battery terminals are clean and the bolts are tight. You can clean them with Bi Carbinate Soda.. Mix 1 teaspoon of Bi Carb to aproxx 2 cups of water and pour over the Battery Terminals and then dry with a thick rag. Gloves should be warn. If its still on after that procedure then look for loose wires but it's possibly better to take it to a battery place as a wrong connection could cause severe damage to your wiring..