A single aftermarket pipe means that the bike has a full exhaust system, header, midpipe and exhaust muffler at the end. Aftermarket systems flow more air than the restrictive factory exhaust. The bike manufacturer has to meet EPA and other government regulations for emissions and noise, aftermarket systems are marked for track use only so they don't need to meet the regulations, so they can concentrate on making power.
There are aftermarket systems with dual pipes, but that's for looks more than power gains.