Strut tower braces are a set of caps that bolt on to the top of your MacPherson strut tower upper mounting bolts, and are connected across the engine compartment by a rod. These devices are used to stiffen your car’s body against rolling while going through extreme maneuvering. G-forces generated during hard-cornering maneuvers are transmitted through the strut tower to the brace and across the connecting rod to the other tower, thereby reducing body roll. These are most effective when used in conjunction with heavy duty aftermarket roll, or anti-sway bars. Pictures of a few examples can be seen here.
For more info, see What a Swaybar does