An X-Pipe exhaust system is a type of dual exhaust which allows for a balancing of the back pressure and smoothing of exhaust flow by causing the exhaust streams from both headers to mix in an opening on the inside x-shaped piece of exhaust pipe. In order for your car to run its best and produce the most horsepower, the exhaust flow from the exhaust port to the end of the tailpipe must flow as freely as possible, and as equally from each port as possible. High flow exhaust systems provide the flow while the X-Pipe shortly after the headers balances the flow between the two sides. Examples can be seen here.
For more info, see How Exhaust Systems Add Power