When the player clears certain stages, he/she will find a rainbow bubble. All he/she must do to use it is flick a Kirby at it. Once the Kirby pulls it down to the ground, the bubble will burst and its rainbow core will rise into the air. The core will shed its seven colors and vanish, and the colors will create a rainbow bridge. The Kirbys will immediately run across it.
Three rainbow bubbles exist on each of the Popopo Islands (with the exception of Necro Nebula, which is completely devoid of them). When a rainbow bridge is created, the cloud covering the stage-selection screen will shrink -- thus, stages will be unlocked. Once the Kirbys have created the third rainbow bridge on a given island, that island's boss can be challenged.
Watch for these bubbles. Pop them to move onward.”