I think it refers to a box-shaped building housing multiple stores on multiple floors, with store fronts facing outside, as opposed to "strip malls" lined along a street with display windows facing the street.

In the United States, developers who create malls tend to make them alike. They also tend to get into contracts with the same chains. As a result, malls in Boise tend to look very similar to malls in Sacramento or Phoenix or anywhere else. It's as though one took a mall out of a box and put it in the place you want. (Just as one can take a lamp out of a box and put it in the place you want. If they're mass-produced lamps, they tend to look alike.)