Health is the meter of stamina in all versions of Minecraft. It measures the damage taken by mobs, TNT, or other things that could kill or hurt the player (only during Survival Mode and Adventure Mode).
The health bar is located beside the Hunger Meter, and below the Experience Meter. When your Hunger Meter is at 9.5 or completely full, you will slowly regain your health. The speed that the player's health regenerates varies depending on the Difficulty the game is on.