A shrub is a fruit and vinegar syrup that is a popular mixer for craft sodas and cocktails in bars. The combination of fruit and vinegar has roots dating back hundreds of years, when fruit – especially berries, since they have a very short shelf life – was mixed with vinegar to preserve it. Soaking the fruit in vinegar extracted flavor from the fruit and, while the fruit was still edible, the vinegar tasted a lot better than it did before the fruit was added. Somewhere along the line, sugar was added to the fruit and vinegar syrup. It was served as a tonic (vinegar is great for digestion), mixed with water or soda water to create a beverage, and mixed with spirits to add some sweetness to them.
Shrubs have surged back to popularity with the craft cocktail movement, since they’re incredibly flavorful and very easy to make. Bartenders can do a lot with them, adding a little to infuse just a hint of a fruit/herb/vinegar flavor or using a more generous amount as the base of a drink. And you can still enjoy them the old fashioned way, mixed with water or soda water and get a flavorful drink that will settle your stomach at the same time.