A flavored soup, sauce or gravy base; also refers to the powdered part of modern instant vegetable and noodle soups, etc.
Traditionally, homes kept huge pots of liquid simmering on their wood-stoves all-year-round. Instead of discarding vegetable peelings and tops, or throwing away bones and trimmings of meats, these were added to the "stock pot". Water was continually added, as necessary.
When a soup or sauce was being made, the cook would strain some of the liquid from the stock pot through muslin (or similar material) and then dilute and season this liquid appropriately for the desired recipe.
With the demise of always-hot wood-stoves in developed countries, dehydrated concoctions of herbs, spices, vegetables (and occasionally meats and fish) were created to replace the invaluable 'soup stocks'. Powders, cubes and thick pastes are all very popular.