When a mercenary is stuck in the dark and needs light fast, they can use flares to brighten the area up some. The flare produces dynamic light that is fairly short-lived. It lights up with a radius of a few meters.
Flares can only be used by ARGUS PMC mercenaries in the online multiplayer mode of Splinter Cell: Chaos Theory. They are typically used for finding the SHADOWNET players who hide in the shadows and try to go unseen. They can come in great use at times, but they help give away the users position as well.