The CD64 is a game backup device made by UFO/Success Company for the Nintendo 64 that allows users to run ROM files off a CD-ROM attached to the system.
Similar to the Doctor V64 and the Z64 units for the N64, it is most commonly used for playing backups of Nintendo 64 games.
The CD64 sits underneath the N64 unit, making use of the EXTension port on the bottom of the console.
Once connected and plugged in, a game cartridge is taken out of the N64 and one is placed into the slot on the face of the system, just underneath the N64 and just above the CD-ROM drive, to act as a kind of boot disk.
When the unit is turned on, via the N64 power button, a GUI is shown where you can choose to launch the game in the slot, or run the ROM file on the CD in the CD-ROM.