An uninterruptible power supply is sometimes referred to as a battery backup. It is essentially a device which maintains a continuous supply of electricity to certain equipment such as your server unit in the event of a power failure.
You can normally receive about 20min of continuous power after a failure. The idea of the standard system is not to allow you to keep working but more to allow you to safely shut down your computer system. Ideally you would install this device with interconnecting software that automatically shuts down your system for you. This is particularly useful when it happens overnight or on weekends.