You need water to live! Your water drains more quickly in the heat of the afternoon, when standing in direct sunlight, or when exerting yourself. If you run out of water, you'll start losing Health instead. Water may be poisoned or corrupted, but can still be bottled and drank in desperation.
Without food, you'll starve! Most food comes from the seeds and fruit of plants, and can be cultivated to grow more. If you run out of food, you'll start losing Health instead.
Your ability to survive damage from the many threats in the world. Health recovers slowly if you're well fed and watered, and faster from certain foods and medicines. If your Health reaches 0, you'll die and become a ghost - but can be reborn at your home spring.
The personal energy you use to work magic - it recovers fairly quickly if you're healthy. You'll use this to power many artifacts, tools, and abilities.
A spiritual energy given off by all living things. You can collect Essence from living plants as night falls, and use it to commune with spirits or in magical crafting. The more Essence you hold, the faster your Mana recovers.