From the Super Mario Wiki
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Cloud Cannons are objects that appear in Super Mario 3D World. When a player jumps into one, the cloud will produce a white Cannon Pipe that shoots the player into a Coin Heaven, which usually gives the player a Super Star or a Piranha Plant used to run through a group of enemies and receive a Green Star at the end. Clouds Cannons are usually only accessible by Cat Mario.
A Cloud Cannon first appears in Mount Beanpole where the player must activate a Cat Wheel to reach it. They seem to replace the rainbow-colored Note Blocks that appeared in Super Mario 3D Land which take the player to Coin Heaven.