Actual keeps backups of your data locally. If something disastrous happens, you can always load a recent backup to get your data back.
Currently it keeps up to 10 backups, one per day of usage of the app, multiple backups of the current day. The result is you will have data backed up to the last 15 minutes, in addition to the last 10 days you used the app.
Backups are only available on the desktop app currently.
The app will reload with the data from that backup. If you want to keep using that backup, you don't have to do anything else, just keep using the app. If you want to go back to the previous data, open the backup menu again and select Revert to original version. This option will be available until another backup is made.
This will force a backup to be created right now. Do this if you are going to do something that you might want to revert later (and don't want to use undo).