Lightroom has three files it depends on: the Lightroom database (.lrcat), the previews database (.lrdata) and the image files themselves.

1. You need to make sure you have backups of the image files themselves, since LR does not create copies of these images, nor does it edit or change the images in any way. If you lose these images, LR can do nothing to recreate or restore your image.

2. The Lightroom database (.lrcat) is where LR stores all the edit tasks, history, tags, collections etc, so it is fairly critical. If you lose this database, you lose your edits and settings, but NOT your images.

3. The previews database is not very critical, but is useful for time savings and performance, but can be easily recreated by LR. If you lose this, LR will simply recreate it.

Personally, I backup my images and the LR database (.lrcat), but not the previews.

Note you can also ask LR to create '.xmp sidecard' files, which essentially create a file for each image that contains the edit 'recipe' for that image. This way you can ensure that edits for an individual image are retained in the same directory as the image itself, making it simpler to backup the edit recipe. If you use .xmp, be sure to back these up as well, and also note that the .xmp does not store all edit history, just the last edits.

A backup is only part of the task, but a good first step. Make sure you cover all possible doom scenarios: having an external hard drive backup of your images, that are stored on you PC is fantastic, until a fire/flood/burglar takes both. So, in this case, be sure you have ANOTHER backup in an offsite location.