Hot-swappable is the capability of being able to disconnect and connect devices while the computer or another device is on and have those devices be detected without having to reboot the computer or device. For example, eSATA, FireWire, and USB are examples of interfaces that are hot-swappable on computers.
In the picture is an example of a hot-swappable network drive being swapped out while the other drives continue to be used.
Note: Not all computers support hot swapping, and not all devices are hot swappable. For example, PS/2 devices are not hot swappable.