These are "Container Installable Archives," which are installed via the FBI app to appear directly on your 3DS home menu.
This homebrew tool is essential for installing games (CIAs) directly on your 3DS. Nintendo 3DS Roms
Use the definitive 3ds.hacks.guide for up-to-date, safe instructions on installing Custom Firmware (CFW) such as Luma3DS . SD Card: A 32GB or larger SD card is recommended. These are "Container Installable Archives