The idea is to connect other devices to the GameBoyTM. Usually this can be accomplished by using standard input/output interfaces called ports. These ports are 'pins' to connect external devices to data-, address- or control-buses. The parallel port should be accessed as easily as a RAM or ROM, to keep programming simple.
The GameBoyTM already has a serial port which could be used if there weren't two major problems. The first is that the data flow-rate on serial ports is slow. The second is that the 'game-link port' does not seem to support any standard protocol for data interchange. Further there is very little documentation on this 'link port' (see gbspec.txt)
Figure 6: GameBoy with adapter & ROM