All flavours of Cumulus (1, 2, and MX) create a sub-folder called backup. Within that is another sub-folder called daily. In these folders, further sub-folders are created, each named by a string of digits representing the date and time the directory was created (without any punctuation) e.g. 20060519090000.
Inside each of these lowest level directory , you will find copies of some of the files from the data sub-folder. Which files are included does vary depending on which build you are running, but generally it is the files that are just about to be updated. The purpose of these backup copies of the files is simply to enable Cumulus to be rewound back to a previous time, and is useful if there is a problem. If you do such a rewind, you do lose data, while Cumulus is running it can read from your weather station in real-time; after a rewind all it can get from the weather station is any readings (at whatever interval is available) stored in any logger either that is part of, or is connected to, your weather station.
The directories within the daily sub-folder backup/daily represent start of day position and are created during the end of day process. Be aware that the files do not instantly contain full contents, so if for any reason you need to read them (as I do in my end of day batch procedure that Cumulus starts), you should wait half a minute or so after their creation.
The directories, created within backup directly (instead of within backup/daily) are used to store the same files as they are when Cumulus starts.
Cumulus has a mechanism that deletes older directories when new ones are created. Consequently, it is useful if you are backing up your Cumulus installation (as you should do to another device) that you ensure these folders are backed up at least once a week.