Flashed is a term of being exposed to the detonation of the XM84 Flash Grenade. Because of the Flashbang's properties, anyone who looks at it would have their screen turn into bright white, unable to see anything that's currently on the screen. After the initial white screen, the last thing the player saw before being flashbanged will stay on the screen overlapping their view, thus making it difficult to discern where they actually are. When the player is "blind" at this time, that means that they are Flashed. Most flashed players tend to crouch when blinded to make themselves as small as possible (though its a useless tactic to do in small maps), and fire uncontrollably at the direction they presumed where the enemy was, or they would bunny-hop and fire in circles, trying to kill the enemy.
Usage: Flash grenades are generally not "noob" friendly and should only be used in team gameplay. Using flash grenades can be far more difficult than using frags such as the M67, or M69HE. Usage of the flash grenades could more than likely get you called a few names, get kicked from Elite Moderator rooms, but they can be an extremely effective tool which could turn the tides of the match, even more so than frags or gas/incendiary grenades.