Export is not a backup. You must make exact copies of the original photos, AND you must make a backup of the catalog file.
Making a backup of the catalog file is done withing Lightroom, by going to Catalog Settings/General, and then selecting the proper backup frequency that makes the most sense for you.
Making a backup of the original photos must be done outside of Lightroom, there are many third-party utilities that can do this (some of them are free).
All backups (catalog file and photos) must be done on an automated and regular basis. Making backups manually is not good enough, you will forget or be too busy, it happens to all people!
All backups (catalog file and photos) must be stored on a different physical disk than the originals. Note the word "must". It is not a backup if you store the newly created "backup" files on the same disk as the originals.
CLARIFICATION: Can one use iMac's "Time Machine" to back-up the photos. Thanks
Your images are simply in folders on your computer. They are not somehow mysteriously ingested into the Lightroom program. You can use any backup method you want to backup your images. Just make sure that you do it. There is provision in the Lightroom program to backup the catalog because that is the only file that Lightroom creates and opens.