The Invisibility Booster is a feature Arthur Weasley built into the Flying Ford Anglia. Harry and Ron use it to fly to Hogwarts unseen, but it fails halfway through.
From the Story[]
"Arthur Weasley adds an Invisibility Booster to the flying Ford Anglia, activated by a small silver button on the dashboard. The Invisibility Booster makes the car temporarily invisible, allowing it to be flown without the risk of Muggles seeing it."