Cold-brewed iced coffee is said to have a sweeter, smoother and less acidic taste than the standard brew. It's made by steeping coffee grounds for several hours in room temperature water before they are strained.
Most cafes and restaurants brew their coffee normally and then pour it over ice to produce iced coffee because the cold brewing technique is rather time intensive (it can take between 12 to 24 hours). Because of the time requirements, Starbucks is preparing their cold brew in small daily batches, and is selling it "while supplies last" (meaning locals need to hustle over there before they sell out!).
Some Starbucks locations currently sell the Toddy Cold Brew System, which claims to make "robust coffee concentrate with less acidity." It also makes tea (both hot and cold) and is no-electricity-required!