The 3-2-1 rule is a general guideline for creating backups. It states that you should have at least three copies of your data stored on two different media, with one copy kept off-site. For example, you can have one copy of your data on your computer's hard drive, another on an external hard drive, and a third copy in the cloud. If one copy is lost or corrupted, you will still have two other copies to fall back on.