5-up works the same as a 1-up, but it gives you 5 lives instead of 1. If you have 10 or more lives, it gives you 10 lives. In it's debut has a "+5" on the center circle on its head. Later adaptions changed the color and look of the 5-Up Mushroom.
The 5-Up Mushroom appears in Super Mario Bros. 10. It is very rare, compared to the other kinds of 1-Ups. This time, it just gives 5 lives, not 10 if you have 10 more lives.
Though the 5-Up Mushroom don't appear in the NSMB 5-Star Adventure games, an item called the 5-Up Star appears with the same function.
Gives Mario/Luigi 5 Extra lives.