The Backup widget, found inallows you to restore a previous version of your project file.
In case something went wrong (corrupted project file, unwanted change, …), you can now restore a previous version of the file using this feature. Just select the version you want and click.
The backup files are automatically created each time you save your project. This means that if you save your project every hour, the backup widget will show you a list of all the saved files, with a small image of the timeline at the time you saved the project.
Kdenlive keeps up to 20 versions of your project file in the last hour, 20 versions from the current day, 20 versions in the last 7 days and 20 older versions, which should be sufficient to recover from any problem.
More details how to get backups when the project file version is upgraded see Project File Details.