Online backup

Online backup is a service (often free) which allows you to upload any of your files for storage on their servers.
There are usually limits to time kept or file size when free.

I suggest you search Google for online storage and examine different sites which are shown. Alternatively, add reviews to the search terms to find out the different allowances and capabilities offered by different suppliers.
Never forget the cost to your ISP account of the bandwidth used for uploading all your files!! (and the time taken).
The tewchnique is that you end up with a copy of your datafiles (usually), just as if you had fitted a new hard disc to your computer and copied the files to that (that would be offline backup).
The only requirement to have online backup is that you have an internet connection and have subscribed (maybe freely) to one of the online backup services.