This is an Add-on for RAMPS boards that allows you to connect a micro SD card to your controller.
You can upload G-code files to the SD card using a card reader or throught host software like Pronterface or Repetier.
You can then execute the programs from the host software. This can be very helpful if you have a slow PC.
The printer will never have to stop and wait for the next instruction from the PC.
Mounts directly onto the AUX-3 pins on the RAMPS board.