The Flashbang is a grenade that deals no damage but blinds any player that sees it when it explodes. The length and severity of the effect depends on the players viewpoint and model orientation. If a grenade ignites behind you, the effect will be minimal. A flashbang going off in your direct field of view, however, will leave you completely blind for a very long time. The flashbang is a difficult grenade to use effectively but plays a crucial role in securing and defending angles and bombsites.
Players should consider the layout of a map when they want to use flashbangs. Angles that opponents often have pre-aimed (e.g. the corner at Long A on Dust 2) are common targets for flashbangs, as an experienced player can predict an opponent's position and throw the flashbang into their expected field of view. That being said, just throwing a flashbang into the open is not always the best approach. If a flashbang appears on a player's screen well before exploding, the player in question has plenty of time to turn away, mitigating the effect of the flash in time to train their crosshair back on the angle you plan on peeking. The stronger approach is known as a "pop-flash": bouncing the flashbang off of walls and other obstacles in the environment so that it explodes just as it appears on the opponent's screen. When performed properly, the opponent should have no time to react before they are blinded. Throwing the grenade underhand (mouse 2 by default) can achieve a similar effect. This is particularly effective against AWPers.