It is an elaborate form of balsamic vinegar, the local variety Grenache.
 
Please note that a balsamic vinegar reduction made from the juice of the grape until it becomes a kind of syrup. This aged for a long period of time, which is mixed with vinegar younger and racked boots of different types of wood.
 
The result is a dark vinegar, thick and shiny, with a complex flavor, sweet and sour at the same time.