Nothing. It was originally put in by Nintendo with the anticipation that something would go there as an add on later that they could sell. In Japan the only thing was the Stellaview but that never came to America. They later added Extention ports to the N64, again only in Japan this is the 64DD short for Disk Drive and didn't leave Japan due to it being a commercial failure. Other EXTention ports can be found on the Gameboy, Virtual Boy, and Gamecube. All with the idea of system connectivity and post-release peripherals.

To reiterate, it doesn't do anything. Not in NTSC anyway.

Was originally planed to have a cd console attached like the Sega cd. They first worked with Philips (giving us 3 zelda games and a mario game on the Phillips cdi), than they worked with Sony. Sony broke away and gave birth to the Sony PlayStation.