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. Not much else is known about Stock Balls, but Sakurai suggests to not leave any Stock Ball behind.
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.
 Trophy Description