An old bottle of "Kuyavian Stomach Essence", bitters from Posen, Germany (now Poznań in Poland).

A bitters (plural also bitters) is traditionally an alcoholic preparation flavored with botanical matter for a bitter or bittersweet flavor. Originally, numerous longstanding brands of bitters were developed as patent medicines, but now are sold as digestifs, sometimes with herbal properties, and as cocktail flavorings.

Since cocktails often contain sour and sweet flavors, bitters are used to engage another primary taste and thereby balance out the drink and make it more complex, giving it a more complete flavor profile. (Full article...)