From the Super Mario Wiki
♥s are the collectible points in the video game Yoshi's Story. They are symbols of happiness and essential to improving Yoshi's Mood. They also recover petals on the Yoshi's Smile Meter. ♥s are earned by eating Fruit, defeating enemies, and collecting special Heart Coins. Additionally, one hundred extra ♥s apiece are awarded for each Melon eaten, each Special Heart collected, and for each Baby Yoshi that survived the current stage.