The Famicom Disk System RAM Adapter connects to the cartridge slot on a Famicom, and is plugged into a Famicom Disk System with a hardwired proprietary cable.
The RAM adapter contains, like the name suggests, RAM which game disks are loaded into. From here, the games run like normal.
In addition to RAM, it also contains a custom disk controller chip, the RP2C33, which takes care of talking to the disk drive and also contains the FDS BIOS. The chip also provides 1 arbitrary wave expansion channel, which gives many FDS games a unique sound.