A flare is a device used throughout the world to illuminate environments at night, signalling and as countermeasures to heat-seeking missiles.
In Armed Assault, the flares the player has access to are Star Parachute Flares which are fired from an under barrel grenade launcher, like the M203, and slowly drop back to earth with a parachute extending illumination time.
They can be used at night to illuminate areas if friendly troops do not have night vision equipment or to signal friendly helicopters for a pick up. Be careful, if used during the wrong situation, a lit flare may give away the player's position and not the enemies. They produce little effect during the day.