Monstro's Belly Brawl
From Kingdom Hearts Wiki: A world of information not accessible by Gummiship
The objective of the minigame is to defeat Heartless until the meter on the screen is completely filled. The meter increases with every Heartless defeated and decreases with every second the player doesn't defeat a Heartless. When the meter is filled and all of the Heartless are destroyed, the minigame is over. Monstro then sneezes Sora out of his body and the world is completed. The minigame has to be completed in order to go on with the game.
If you fail to complete the task and the meter becomes empty, you simply leave the Key to Truth room and must return to take the challenge again.