I am flattered that you should ask me only.. there may be better persons on the site who are electrical engineers.
Well, to put it simply IGBT is a transistor like device, which you will find in your TV, Fridge and other higher current using devices. The IGBT is rated at over 600 KVA, and therefore it allows current to pass through it if it is allowed to do so, depending upon the end use. In railways it allows excessive current (as opposed to voltage) to pass through without heating itself up (the IGBT). Thus electrical circuits on the OHE supply of 25KVA (25 thousand volts) are simply transferred by the transistor device to the wheels depending upon the notch position of...
more...

the locomotive controlled by the Loco Pilot. Since it has a very sturdy structure and requires less attention, just like your fridge not going out of order quickly, it helps transmit power to the wheels much faster than other electrical devices. Further it is sturdy enough to withstand cold, hot and dust from surrounding areas. In order to maintain its property properly, IGBTs are also provided with cooling fans to dissipate the additional heat generated by the control of pass through current through it.
Others may please chip in and correct me. I would like to be educated as well.
A single line answer is not possible unfortunately.