You can earn up to five stars on your save file through varying degrees of completion of the main story mode.

Stars	How to unlock
1 star	Beat Bowser in story mode.
2 stars	Unlock and beat every level outside of Superstar Road.
3 stars	Get every star coin outside of Superstar Road.
4 stars	Get every star coin in the first 8 levels of Superstar Road to unlock the last level.
5 stars	Get every star coin in Superstar Road-9.