Backup appliance is a type of data storage device / equipment that accumulates the backup software and hardware components within a single device.
It is a type of turnkey and all-inclusive backup solution that provides a central interface for backup processes, tools and infrastructure.
Techopedia explains Backup Appliance
Backup appliance is a hardware device that is preinstalled with backup management software, storage drives, network interfaces / ports and other backup administration utilities.
It works by being connected to the devices/ components on the local/ organizational network. The pre-installed backup software captures data / files from each connected and configured node / device and stores it on its local storage media.
The same data can be replicated /restored through the backup appliance when required. It can also be connected to external storage / backup facility such as SAN, NAS or cloud backup.
Moreover, it may also provide data security and protection services by encrypting data at rest and restricing access to the appliance to only authorized users.