Sauerkraut is pickled or fermented cabbage that has a distinctive sour flavor. If you see someone spooning something pale yellow from a jar onto their hot dog, chances are that is sauerkraut.
You may have eaten sauerkraut on a hot dog or in a Reuben sandwich. It's also common in German, Czech, Polish, and French dishes, and in slightly different forms sauerkraut shows up in the cuisines of many other countries and regions. Sauerkraut is preserved and fermented by specific lactic acid bacteria that contribute to its sour taste. In German, Sauerkraut means "sour cabbage" or "sour vegetable."