Basics
The jumper pak looks nearly identical to an expansion pak and serves no function other than to literally take up the slot that the expansion pak would normally fill. The expansion pak was not released until 1999, four years after the initial release of the system and was even then simply an optional add-on --- though certain games such as "Donkey Kong 64" required it.
Importance
The jumper pak served to fill the sockets that would be taken up by the expansion pak until the user upgraded. It was necessary to eliminate the RAMBUS memory, which could not have run with the 4MB of memory the system had without the expansion pak. The console would not play games without either the jumper pak or expansion pak. Trying to do so would result in a blank screen.
Expansion
The expansion pak gave the system 4MB of additional memory, bringing the console to a total of 8MB. The pak replaced the jumper pak and allowed games to expand on their graphical prowess as well as open additional levels and features. Certain games, including "Perfect Dark," ran with reduced graphics without an expansion pak while others, such as "The Legend of Zelda: Majora's Mask," could not run at all without it.
Installation
The jumper pak and expansion paks can both be swapped by opening the lid near the front of the top of the console. A red label will mark the top of the jumper pak. The jumper pak is removed with a removal tool included with the N64 expansion pak or with a spoon. The expansion pak is then pushed into the slot the jumper pak had once taken up.