Ransomware often encrypts or deletes the backups it can reach on the network along with your files. An immutable backup is stored so that, for a defined period, neither malware nor an administrator account can change it.
An offline (air-gapped) copy gives similar protection — for example, a disk disconnected from the network after the backup runs. Both are your last reliable chance to recover after ransomware.
Example
Backups in cloud storage are locked for 30 days — after a ransomware incident the company restores from the last clean copy.