Hardware device used to supply power to computers and peripherals for a short time if the power happens to be low or removed. A battery backup allows the computer to shut down safely or keep the computer running if the power is off for less than a few minutes. The picture is an example of a battery backup from APC.
Businesses often use large battery backups to safely shut down servers if the power goes out or keep the servers powered if the power is out for a few minutes. These backups are essential if your server is running critical software, such as a customer database.