Miso is a sort of femented soyabean paste, salty-ish, used to give a deep savoury flavour to Japanese stocks, soups and broths. It's a textured paste, so when you dissolve it in hot water it does have little bits of the fermented beans in. I love it - it makes a delicious soup. Try looking on the Clearspring website or, if you have a local Sainsbury's near you, you can usually find it in their 'Special Selection'. It's not cheap, but a little goes a long way. If you can't source Miso and your recipe uses it as the basis for a stock, you could substitute Marmite or Vegemite, but sparingly.