The Flying Nimbus is a small magical yellow cloud that can fly at high speeds. It can carry people who are pure of heart.
Korin possesses a single large nimbus which can be broken down into smaller clouds such as the one Goku possesses. When the original Flying Nimbus is destroyed by the demon Tambourine, Goku is therefore able to acquire another piece.
This second Flying Nimbus is passed down through Goku's family and makes many apperances throughout the Dragon Ball Franchise.