Bitrot Detector is meant to be used along with a backup system—any backup system. That way, when Bitrot Detector finds unwanted changes to your files, you can restore the original, unmodified files from your backup system, while there's still time.
Bitrot Detector will work with any backup system, whether you backup your files to an external drive or to an online service. If you have yet to implement a backup system, we'd recommend a backup service, so that your files will be backed up continually, without the need to remember to perform backups.
Here are some helpful links as you choose a backup service:
There's No Excuse for Not Backing Up Your Computer. Do It Now.
Five Best Online Backup Services
Comparison of Online Backup Services (Wikipedia)