HP-Up Hearts (known as HP Plus in the British English version of Paper Mario: Sticker Star) are special hearts Mario can find during his adventure in Paper Mario: Sticker Star and Paper Mario: Color Splash.
Paper Mario: Sticker Star
In Paper Mario: Sticker Star, these hearts will increase Mario's maximum HP by 5 and will fully refill his health bar. It also increases the attack power of his First Strike by one for every two collected. There are 16 HP-UP Hearts to find and by collecting them all, Mario will have a total amount of 100 HP.
Paper Mario: Color Splash
In Paper Mario: Color Splash, there are six of them and each time Mario and Huey collect a Big Paint Star, their maximum HP is increased by 25 and the power of the First Strike is increased as well. When all six are collected, Mario reaches a grand total of 200 HP.