The 3-2-1 backup rule is a widely used guideline: keep at least three copies of your important data, store them on two different types of media, and keep one copy offsite. It is designed so that no single event, such as a drive failure, fire, theft or ransomware, can wipe out every copy.
In practice the live data is copy one. A local backup to a network drive or backup appliance is the second copy, and a cloud backup or offsite copy is the third. Many modern versions add an immutable or offline copy that attackers cannot encrypt or delete, sometimes written as 3-2-1-1.
Retailers and hospitality businesses apply it to data that is hard to replace: accounting files, back-office and POS data not held in the cloud, customer and supplier records, product images, rosters and the configuration of servers and network devices. Cloud systems still need thought, since a software provider's uptime is not the same as a backup of your data.
The most common failure is a backup that has never been tested. Check that backups run, alert on failure, are encrypted, and can actually be restored in the time you need. A drive left plugged in beside the server may be encrypted by ransomware along with everything else.
It is simple, memorable and effective, and it underpins any disaster recovery plan.