It adds fresh air to the hot exhaust to burn off excess fuel in the exhaust (an environmental/emissions thing). With factory mufflers on, you don't hear the pop, pop, pop on decel very much.. HOWEVER if you change to aftermarket pipes, the pop's are very noticeable. Bypassing/removing the AIS does nothing more than "clean" up the look and remove the pops.
If you are planning on taking it off, make sure you don't need to pass an emissions test for your inspection. It will fail without the AIS