To be clear: when you import a picture in Lr, it simply notes where the photo file is on disk (and generates a viewable version of it called a preview, but that preview is NOT the photo file itself), so your picture files are not in Lr, Lr just knows where they are on disk.
So, you need to back them up from wherever they are on disk, to a backup location on another disk, and to an offsite location too if you're worried about fire..
If you are not sure where a photo is on disk, you can context-click it (right-click in Windows, ctrl-click on Mac), and choose "show in Explorer/Finder" (there are other ways too).
Your edits are in the catalog, so back that up too.
Exporting simply make an edited copy - no need to back up exported files, just catalog and photos files. You can always recreate exported copies if you have the catalog and requisite photo files.
After you learn the ropes, you may also want to save/backup (xmp) metadata, but that's getting too far ahead now..
Rob