This feature allows you to apply an encryption to the resulted backup archive using an encryption key. Later you may decrypt that archive on any system that provides OpenSSL support.
Note that the Advanced Encryption Standard (AES) is the one of most secure cipher adopted worldwide by, among many others, the US government and their National Security Agency (NSA).
It is worth mentioning that when you want to restore the encrypted archive using this application the decryption is done automatically.
The main advantage of OpenSSL over Mcrypt implementation (see below) is that the former one is much faster than the Mcrypt. Some tests show that OpenSSL can encrypt even 20x faster than Mcrypt and decrypt about 30x faster than the last one!