From the Super Mario Wiki
A Stock Ball is an item in Super Smash Bros. Brawl. In the Adventure Mode of the game, the Subspace Emissary, Stock Balls increase the number of total lives the player has left.
Each Stock Ball is surrounded by a bright purple and yellow glowing aura. In the center is what seems to be a human silhouette, indicating the extra life.
||Wii Super Smash Bros. Brawl
||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.