The 3-2-1 rule means: 3 copies of your data (the original plus two backups), on 2 different types of media (e.g. a NAS and the cloud), with 1 copy in another physical location.
This way a single incident — fire, theft, disk failure or ransomware — can’t damage every copy at once. Today one more condition is often added: one copy should be offline or immutable.
Example
The original is on the office server, a copy on a NAS and another copy in the cloud that can’t be deleted from the network.