The Nintendo 64 Controller Pak was a memory card for the Nintendo 64 game console. Like the PlayStation Memory Card, it was used to save game data and this could be shared with other Nintendo 64 owners.
The Controller Pak plugged into the controller rather than the console itself, and held 256 KB, split into 123 pages with a limitation of 16 save files. Data was not saved on flash memory, but on battery backed SRAM. Bigger memory cards were made by third parties and ranged from around 1-4 MB in size.
Many first-party games typically used on-cartridge storage to store game data. It was mostly third-party games that used the Controller Pak, to save the expense of including memory in the Game Pak itself.