The Joint Test Action Group, or JTAG, is the name given for the IEEE 1149.1 Standard Test Access Port and Boundary-Scan Architecture. This architecture has been used in most computer processors since Intel released the first processor with JTAG, the 80486.
This IEEE standard is what defines how computer circuitry is tested to validate it is working properly after the manufacturing process. Tests are run to verify solder joints on the circuit boards, ensure there are no imperfections in the circuit board connections, and no issues with the bonds & bond wires that connect integrated circuit pads with pin lead frames. JTAG provides testers with a pins-out view from each integrated circuit pad, helping to identify any faults in a circuit board.
A JTAG interface can be added to a chip. This interface/port can be used to connect a probe to the chip, allowing for a developer to manipulate the chip and its connections to other chips. Developers can also use this interface to copy firmware to non-volatile memory within an electronic device.