Multicarts are games that have several games inside of them kind of like a 100-1 game. Most of these games are made by one person and then put on the Black Market, or they might end up in flea markets (please realize that it is illegal to make Multi-Carts).
Inside every game there is a chip called an EPROM (EROM). This chip has the entire game's program and graphics, programmed onto it. When everything is in there though it only takes up 2% of the chip's memory (a waste if you ask me!!) so someone takes the cart and reprograms the chip to store more than one game on it, and they load up the chip's space with other games and then they program in a menu program like this..in QBASIC language.
And youre on your way to making multi-carts!! The "CALL" statements executes the game when you select that number. BTW, this i sjust a basic way of doing it they are better ways of doing it and you shouldn't try to do it this way cause it wont work with teh Nintendo source code.