JTAG – (Joint Test Access Group) is a tool used to read and write the firmware stored on the flash chip of many modern-day electronics. it’s actually a testing/troubleshooting interface built into the the motherboard at the factory. Hobbyists have been writing their own software so they can interface with these devices using their computers.
JTAG can also used to repair a router when it has been “Bricked”. This usually happens when you loose power in the middle of a firmware upgrade or flash an image that is incomplete. Using a JTAG, you can rewrite the flash on a device that is normally stuck in some sort of boot-loop or power-cycle. I’ve repaired several devices that would have otherwise been trash using a JTAG.