This is San Francisco's shopping district. Some of its many high-end stores include Neiman Marcus, Bloomingdales, Saks Fifth Avenue, Tiffany & Co., and Giorgio Armani. The area even has a multi-level Macy's with a men's Macy's across the street! Union Square is also home to more affordable stores, such as H&M.
In the center of Union Square, atop an underground parking garage, lies an elevated plaza that is usually bustling with tourists and shoppers. This is what's specifically meant when someone says "Union Square" (and it's what's drawn on the map here).