Hot-swappable describes a component device that can be removed or installed without powering down the host computer. For example, eSATA, FireWire, and USB are examples of interfaces that are hot-swappable on computers.
The picture is an example of a hot-swappable network drive being swapped out while the other drives continue to be used.
Not all computers support hot swapping, and not all devices are hot swappable. For example, PS/2 devices are not hot swappable.