kwas in general means that it was fermented. So, in the old days, when women were cooking they always used "kwas" as a base for their soup, if the soup was to be of a sour taste.
Making kwas involved either piece of rye bread or rye flour mixed with water, fermanted in the jar. Nobody does it anymore, because of time limits.
This base was used for beet soup(Borsch) or white borsch(zur), which I can still buy in Polish stores here in Canada. It is also widely available in Poland but only for the white version.
My mother occasionally mixed that "kwas" at home and it made a huge differance in taste.
Oh, I just found a recipe for "kwas" in teh Polish cookbook, so if somebody is interested, I will post it
I would never, ever add vinegar to my borsch, try lemon juice.