CAB file is short for cabinet and means exactly that. It is a cabinet that contains a bunch of files and a set of instructions of what to do with those files when the cab is executed.
Cabs are useful when a device is running Windows CE since it lacks the persistent storage that windows mobile has. What this means is if the battery dies or a cold boot is performed, all applications and data would be lost on a CE device. The cab file can auto launch at the time of boot restoring the applications and settings. Data would still be lost unless they were saving it to a storage card.
Installation of the CAB file will only need to be done once. Then the file will be stored on the device until it is removed.
This is a non-issue on devices with Windows Mobile 5.0 and up.