Hi, the delicious Polish cabbage salad in jars is actually fermented cabbage. We know it here, in the UK, as Sauerkraut, which is the German version. Sorry I can't remember the Polish name. Anyway, it is yummy and a doddle to make on your own.
The shop stuff is okay, but if it's the cheap variety, it is probably pasteurized - effectively killing all the good, healthy bacteria. If you make your own, it tastes better and is much cheaper than the posh, unpasteurized versions.
I recently bought two cabbages for 38 pence. I got two large jars of fermented cabbage out of that. Just Google 'Better done yourself' or search directly on YouTube.