This is the Nintendo 64 Transfer Pak (official model NUS-019), a peripheral accessory for the Nintendo 64 console released around 1998–2000.It plugs into the expansion port on the bottom of an N64 controller and has a slot on top where you insert a Game Boy (or Game Boy Color) cartridge. Its main purpose was to allow data transfer between compatible N64 games and Game Boy games—most famously used with Pokémon Stadium (and Stadium 2) to bring your Pokémon from the handheld games into the 3D battles on the console, or to unlock extra content in games like Mario Golf, Mario Tennis, and Perfect Dark.It's gray plastic to match the standard N64 controller, with the multicolored N64 logo sticker on the front and the Nintendo logo embossed at the bottom. The three small circles you see are indicator lights or vents.