I recently bought TM4C123GXL evaluation kit (Texas Instruments). I found 2 microcontrollers on that board. I heard that it is something related to JTAG. But I didn't understand what it is even after browsing about it.
JTAG is the standard debug protocol for hardware-based debuggers, allowing setting of breakpoints, single stepping, querying register values, etc.
Some devices use raw JTAG via a dedicated port on the board which a special hardware device like an ARM DStream unit can connect to.
As an alternative many development kits use a micro-controller to manage the JTAG of the ARM processor being used for development. The host PC can talk to this micro-controller via a different protocol, and this micro-controller translates that into the JTAG signals locally on the board. In the case of the TI board it looks like USB is used to provide all of the connectivity to the development PC, including the JTAG connection, so this second micro-controller has to handle the USB connection to the host, and decode / encode packets to talk to the raw JTAG port on the micro-controller being used for the actual software development.
HTH, Pete