A Stock Ball is an item in the Subspace Emissary of Super Smash Bros. Brawl. It grants the player one extra life. The extra life shows the Super Smash Bros. Symbol on the lives column 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. It's prevalence is markedly decreased on harder difficulties. If played on Intense difficulty, the Stock Ball doesn't exist.
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.