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 shutdown 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 shutdown 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.