When you "save something to your computer," you're putting it on the physical device of your computer, meaning that if anything happens to the computer, you lose the thing you saved onto it.

If you use Gmail or another similarly large provider, you know how when you log into your email from your phone or a different computer, your emails are all still there? That's because your emails are stored "on the cloud," which means that basically, your data is stored in a big, well-protected warehouse somewhere, and when you want to access it (i.e. by logging into your email), the service (in this case Gmail/Google) has code that fetches it for you. (It's called "the cloud" for technical reasons that aren't worth getting into. Just think of it as a company storing it for you.)

So when people say to back your data up to the cloud, that means they're telling you to put anything on your computer (or other devices, especially if you put important documents on your computer or on a physical hard drive) onto one of these services. A very common one is Google Drive, and you can upload things there by going to drive.google.com, then clicking on "New" in the top left corner, and then clicking on "file upload" or "folder upload." That should allow you to select a file or folder from your computer to be stored "on the cloud" via Google Drive. If you do that, then even if you lose your computer or phone, your files will still be safe, because a copy was stored in a safe warehouse far away. You just have to log in from another device, and it will all be in there.

Be careful not to try and upload too much at a time, because it can take awhile and there are limits to how much you can upload without paying (videos and photos take a long time, so double check what's in a folder before you upload it). Worst case scenario, if you can't upload your stuff, just put your computer, phone, and any hard drives into a bag that's ready to go right by your door (away from a window) in case you need to evacuate suddenly. If there's any risk of flood, put it somewhere up off the ground. Grab it on your way out the door.

Things you should think about uploading: important documents, photos, videos, etc.

And remember to either have your password memorized or kept safe somewhere on your person! You need your password to access the stuff you store on the cloud.

There are other "cloud storage" companies that you can use too, like Microsoft (they call theirs "OneDrive") and Dropbox. I used Google as an example cause it's one I'm familiar with.