Lights flickered - dead battery maby, or ground wire?

Ok, A while ago i posted about the lights inside my 2002 impala were flickering. Not really flickering, but they would like go dull and back to bright again.

My dad said a few days ago the headlights went off, and came back on. But he doesnt remember if he had to hit the switch to get the lights to come back on or not.

ALso, when you put the key on the ON position, on the little display, it flashes SECURITY for a few seconds, then goes to a picture of a battery, then goes back to SECURITY again.

I dont remember it doing that. DOes htis mean there is something wrong with the security (we dont have alarms) or is the battery going dead, and it is telling us, or what?

Thanks alot!