If you make a mistake, such as falling into a pit, or you tap the bubble button on the screen, you'll pop into a bubble. While in a bubble, you'll automatically glide back toward the beginning of the course you're on. Tap the screen, and you'll pop out and continue running!
In World Tour, you can enter the bubble only twice. If something unfortunate happens a third time, you'll get a game over.
In Toad Rally, there is no bubble button to tap, but you also can't get a game over. You'll just keep popping into a bubble each time!