As you play the game you are almost sure to come across some food. Apples will likely be the first piece of food that you come across. If you press the + button and go to your inventory, you will be able to find the piece of food that you picked up and eat it. In the case of the apple, this will restore a half of a heart. Other foods will restore different amounts of health.
To make the most of restoring health, you should cook your food. Cooking food will return the maximum benefit as the amount of health restored is increased with you cook food in Breath of the Wild. There are numerous different things you can cook, but a simple thing is to just cook the piece of food that you are about to eat on its own. It will usually give a little bit more, but its well worth learning what the best things to cook are.