Its name comes from the meat juices and there are loads of combinations you can do – its up to you. The classic is red wine and beef stock so I’ll describe that one but you can use just stock, stock and another liquid and any other flavourings, just nothing to thicken it.
The classic is to cook a nice and pretty large piece of beef that oozes fat and juices and deglaze the pan with water and/or wine to make a stock. When the pan is deglazed you can reduce it a bit and then strain it into a clean pan. Nothing wrong with adding veg or herbs but just strain it so it is just liquid.
Then you reduce it to concentrate the flavour.
That’s it!