Eating is how you recover lost HP, and is a common way to prepare to fight strong monsters or players. More advanced items will give you extra attack and defense too!
Left click on something that seems edible in your inventory. If it is, you will see an 'Eat' button. Left click on the 'Eat' button, and you will see an eating box on the right side of your screen. This shows you the effects, and how long they will last. In the screenshot below I left clicked on my Algae, it brought up an Eat button. I can now left click that Eat button to consume the Algae.
After you've eaten something, you will see information about how it is affecting you to the right of the main game.
Human bodies eat different things than machine bodies. Keep this in mind when planning an expedition!
Some things may look like they are always better to eat than other things. Looks can be deceiving. Some things will have an addiction chance. If your body becomes addicted to something, you will have a negative stat effect until the addiction wears off, or your body gets more of whatever it is addicted to. Addiction currently comes in 4 levels. A level 4 addiction of a substance is very likely to kill a body.