Deflicker is a setting in Super Smash Bros. Melee and Brawl. It adjusts the way the screen appears when the game is played. When deflicker is turned off, the appearance is sharper, and the screen will flicker a bit. When deflicker is turned on, the appearance of the screen is softer, this is usually the default setting.
Trivia
Most people think the default setting looks blurry and difficult to see.