Health is a statistic measuring the amount of life in your character has.
Food is the act of eating. Preferably, good, healthy food. This can prevent Character Death.
Each Food consumed has different nutritional values and in some case, can even be improved (such as cooking raw meat).
Warning - consuming raw, uncooked meat, while improving your Food Vital, can have a negative impact on your Health Vital.
Foraging or hunting for food will more than likely be a daily routine (at least in the early stages of the game) for your character. As you spend more points on Farming, your character can plant and harvest crops to be turned into staples that can be cooked on a stove or in a camp fire.
Water refers to making sure your character gets enough to drink. Good 'ol H2O. Together with Food, it's the vital part in ensuring your character functions at peek performance.
You start the game with a small amount of water but must quickly find additional water sources. Rivers, lakes and streams can be a good source of water, albeit, maybe not the most clean.
Building a well or a Water Collector will be your best bet in maintaining a steady supply of good, fresh water.