From Minetest

In Minetest, health is measured in hearts (Heart.png). The smallest unit of health is a half heart (Half heart.png). 0 hearts mean death. You can restore hearts by eating food, but you can't exceed 10 Heart.png.

Health is what keeps you alive. If you have 0 hearts, you will die. To eat food, simply click the air with food in your hand as if you were breaking something. Some mods may change the controls and ways of eating, as well as modded hunger bars that are not included in the vanilla stage of Minetest.