it exports the main .exe file which you run the game with along with the necessary .dll files required by the visplayer to run your game & it also converts/compresses the data files (your resources) into a single file or into individual files depending on the export options you choose.
i.e: you could export all of the resources into a single .vis file but it's not recommended as it makes patching your game more difficult in the future - say you've removed some bugs or fixed/replaced something - as it means you'd have to distribute the patch as the newly exported .vis file - whereas if you were to export the resources as individual files then it makes patching much easier as you only need to give the people the files that have been changed; thus saving lots of time, space & bandwidth
Simon, David or Alex could explain it better than I can, seeing as they are the ones wor.king on the code - so if what I've just written isn't clear enough then maybe one of them will answer correctly
P.S: the export is only a generic compiled version of your game - which means it may not be accepted by certain distribution services such as apple mac app store or steam for example. Currently certain things need to be done by the Dev team - namely David - to make it acceptable (something about unique keys or certs or something - I forget)