Miso is made from fermenting soybeans and/or grains with sea salt. It is a popular flavour in many parts of Asia, and is most commonly used for miso soup.
Miso is very healthy because it contains probacteria and Vitamin B12. Great for repopulating your stomach with good bacteria (see Germaphobia) after taking antibiotics (see FoodAsMedicine?), a vegan alternative to Yogurt or Kefir (see VeganKefir).
When using miso, mush up the paste with a bit of tepid water to activate the enzymes (and to make it easier to mix into your food). Miso should never be cooked, as this kills all of the probacteria. Best to add it to your soup after it has cooled to eating temperature. Also, try to avoid adding miso to metal containers or using metal spoons, as the metal de-activates some of the goodness of the miso (see MetalVesselCaution).