A jus is a stock that is reduced until a gelatinous thickness is achieved. The process of reducing the stock simply removes water which intensifies the flavours and thickens the liquid. I am using my kid-goat stock enriched with some red wine and aromatics.
Jus is precious and you will not get a huge yield. It should be used sparingly. It can be used as a thick sauce by itself or it can be poured into ice cube trays and frozen and then used straight from the freezer to thicken and enrich other sauces and gravies.