Discharging is generally referred to when discussing xapcitors like those in a power supply when the method of "recycling" the supply to troubleshoot problems there. You first shut everything down if running and unplug the ac cord.
From there you press the power button like you were going to start the system to see the "caps" in the supply itself drain off their stored charge. Caps are like giant srorage batteries with some. While current is applied they charge up and then discharge what is stored in only milliseconds by reversing current depending what type of cap it is.