Are you like me before i became a better person – when you exit Lightroom and get the question if you want to back up your catalog, your answer is “Not this time”? That’s actually a very bad idea. You ever know when you run into trouble and letting Lightroom spend a minute making a backup is time well spent.
The way you set this up, is you go to Edit->Catalog Settings (or Lightroom->Catalog Settings on a Mac). On the general tab, there’s a dropdown list that lets you choose when to run your Lightroom catalog backup. I strongly suggest you choose “Every time Lightroom exits”.
This backup is of the Lightroom catalog only. It does not replace the backup strategy you need for your actual image files (more on that here). The backup files created are not very large, but Lightroom does not remove these files, so they add up. You should make a habit to remove your older backup files periodically, in order to keep your disk tidy. The backups are stored in a folder named “Backup” under your Lightroom folder. There will be one folder per day. Simply select older folders and delete them from your file explorer.