Answer:
A device or series of devices referred to as "Hot plugging or hot swapping" means that they can be changed out without having to shut down the main system in order to do so.
Think of a USB thumb drive, how you can plug it in, use it, then disconnect it without ever having to reboot the computer. That is a good example of "Hot swap" technology. It is commonly used in RAID arrays or on device programming setups or disk duplicators.