Had to look that up:

The 3-2-1 backup rule is a widely recommended strategy for data protection. It states that you should:

1. Keep 3 copies of your data – One primary and two backups.

2. Use 2 different storage types – Such as an external hard drive and cloud storage.

3. Store 1 copy offsite – To protect against disasters like fire, theft, or cyberattacks.

This approach ensures data redundancy and security, reducing the risk of total data loss.