A signal flare is a secondary grenade that produces an intense light, blinding and disorienting any targets that are in its field of view. It is useful on dark maps in order to blind the enemy, it also can be used to mark positions.
Signal Flares are very underrated. People hate them because they don't blind people, because they compare them to flashbangs in CoD4. Signal flares emit a large glow which makes it impossible to see anything around it, and if you are right next to one, your screen will turn into a kind of red shade which makes it a bit difficult to see. What I like to do with signal flares, is to use them like a smoke grenade. Throw one to make a distraction or to blind an area from view. They are VERY bright, and even if you don't have a red shade on your screen, it can still be pretty hard to see. To avoid this intensity, use the shades perk. (it actually gives you sunglasses )