The Smile Meter is the health meter in Yoshi's Story.
Appearances
Yoshi's Story
In Yoshi's Story, the Smile Meter's first appearance, the Smile Meter appears in the upper-left part of the screen.
When a Yoshi touches an enemy or eats certain things, the petals will gradually fall off. When the Smile Meter has no more petals, the Yoshi loses a life. When falling in an abyss/lava or getting crushed by a compressor, all petals are lost. When a Yoshi eats fruit, petals can be re-gained.
Yoshi's Island DS
In Yoshi's Island DS, Yoshi can lose health while trying to hold on to one of the babies. The Smile Meter is the same as it works in Yoshi's Story.
Other appearances
Super Mario 64 DS
The Smile Meter is also in a Super Mario 64 DS minigame, "Loves Me?", where the player touches the petals to make them fall off. At the end of the game, the flower will then decide.