The ROM gets saved to an SD card. You can also read/write save files to the SRAM, display information about the cartridge on a 0.96″ 128X64 OLED LCD, and calculate the checksum of your ROM dump. You control it using the push button–one click moves the selection down, a double-click moves it up, and a long press executes the current menu option.
As the Maker explains, this shield:
Reads SNES ROMs and reads/writes save games from and to the SNES cartridge–supported types include: LoRom, HiRom, ExHiRom, SuperFX, SuperFX2, SA1 (can’t write save back to SA1)
Read/writes SFC Nintendo Power cartridges
Reads N64 ROMs and reads/writes save games (4K/16K EEPROM + SRAM + Flash RAM)
Reads/writes N64 Controller Paks and also can test a N64 controller
Programs Flash ROMs like 29F016, 29F032, 29F033, 29F1610 and 29L3211 (needs 3.3V)