“The 64DD sat under your N64. Well, not yours — you didn't have a 64DD.”
The Nintendo 64DD (Nintendo 64 Disk Drive) was a machine that attached to a Nintendo 64 and could play games in a disc format. There were also several peripherals used for it. It was released in Japan in December 1999. It was scheduled for release in America in 2000, but it ended up being a commercial failure due to the way it was sold (only through subscription to the RanDnetDD online service), so it was never released outside Japan.
There were four Mario games released on this console, all in the Mario Artist series; there were also several cancelled Mario titles. Overall, there were only nine games released on the Nintendo 64DD, although other games planned for the console were released as Nintendo 64 cartridges instead.
 Mario series games