A bushing is the part that passes through the metal wall of a transformer or an enclosure to bring the conductor outside. Its job is to grade the electric field between the conductor, at full voltage, and the flange, at earth potential, over a few centimetres.
At high voltage that is achieved with capacitive construction: concentric layers of paper impregnated with oil or resin, separated by conducting foils that act as series capacitor plates and grade the potential. That design is what is called a capacitance-graded bushing, and it carries a test tap giving access to the outermost layer.
It is a critical component: an appreciable share of catastrophic transformer failures starts in a bushing. It is monitored through that tap, measuring capacitance and dissipation factor — a punctured layer changes the capacitance in a characteristic way — through thermography of the porcelain and through the condition of the oil. Online monitors track the magnitude and angle of the leakage current of all three bushings at once.