Untethered mode is a feature in 3D printers. In this mode, the printer is fed G-code through a memory card instead of being connected or tethered to a computer. This is specially helpful when one has multiple printers or is printing a model which can take a long time to complete.
It is very much possible to add this feature to your printer, provided you can tweak the firmware and add a SD card reader to the controller board in your printer. The necessary instructions are to be added to the firmware, so that the printer can be guided to read the G-code from the SD card and print accordingly.