A Stock Ball is an item in The Subspace Emissary of Super Smash Bros. Brawl. It grants the player one extra life. The most number of extra lives (prior to the final boss Tabuu in The Great Maze where the player gets six characters) through the first 30 stages is five. But as long as they have the Stock Ball though. The player normally gets four stocks (four characters) but can use Stock Balls to build up their lives from the stage (only up to five). The extra life shows the Super Smash Bros. Symbol on the lives category on the very left. It usually appears after defeating several enemies or completing a puzzle. In game, it can look like one of two different ways- either a Super Smash Bros. symbol or a small headed version of the waiting characters. Its prevalence is markedly decreased on harder difficulties. If played on Very Hard or Intense difficulty, the Stock Ball doesn't appear at all.
The Subspace Emissary is played in Stock mode. When you're defeated, your stock decreases by one. If your stock reaches zero, the game ends. However, you can increase your stock by one by picking up a Stock Ball when you come across one. There's a big difference between having a stock of zero and one, so be sure not to leave any Stock Balls behind.