Sauerkraut is finely shredded cabbage preserved by salting and lacto-fermentation, which gives it a sour, tangy flavor and crisp texture. Fermenting cabbage this way is often traced to ancient China, though the dish most associated with sauerkraut today developed in Germany and Eastern Europe, and it remains a staple of Pennsylvania Dutch cooking in the US, as in the Reuben sandwich.
It's commonly served as a side dish with sausages, pork, and other smoked meats, and is also used in soups, stews, casseroles, and as a filling for dumplings and savory pastries in some Eastern European cuisines. Its sourness pairs well with rich, fatty meats and with apples, caraway seeds, and juniper berries.