From the Super Mario Wiki
Bob-omb Trampoline is a multiplayer minigame featured in New Super Mario Bros.. In-game, it is in the 1 on 1 category, meaning that only two players can play.
The game involves the two players trying to keep Bob-ombs falling into the lava on their screens by having the Bob-ombs land on the trampolines instead. The players control the location of the trampoline by dragging it across the bottom screen using the Stylus. Any Bob-omb landing on a trampoline is sent over to the opposing player's screen. As the game progresses, more Bob-ombs appear on-screen. The player is only allowed to miss up to four times before they lose for the next one.
"Position trampolines to catch the falling Bob-ombs and bounce them onto your opponent's screen! You lose if you miss five Bob-ombs."