Malls are big buildings that take two squares. Once you reclaim one, the only thing you can do with them is make them into fortified malls which gives you 15 extra defense. It is considered as a defensive building afterwards.
"People just don't shop like they used to, so we don't have much use for a mall here, but it might make a good defense post." (unfortified)
"Secured malls add a defensive bonus of 15, just like in the movies" (fortified)