A Stock Ball is an item in Super Smash Bros. Brawl, found only in Adventure Mode: The Subspace Emissary. A Stock Ball is surrounded by a bright purple and yellow glowing aura, and has a silhouette-like image in the center. A Stock Ball, as its name would suggest, gives the player an extra stock upon being picked up. A Stock Ball usually appears after defeating a group of several enemies.
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.