Minecraft/Health

Most games have hit points, health or life to define when your character dies. Minecraft uses "health".

Health
You can see how much health you have by looking at the hearts above-left of your hotbar. Each heart is two points of health.

If a heart turns black, you lost 2 health. If only half a heart turned black, you have lost 1 health.

Dying
If your health reaches zero, you die. This makes you lose everything in your inventory (unless you turned the "keep inventory" setting). It makes you also return to the last bed you slept in, if possible, or the world spawn (where you was when you started the game.