AES is the Advanced Encryption Standard which is the method of encryption used by the U.S. government. 256-bit is the size of the key used to unlock the encrypted data. Most organizations require that you sanitize (delete) all the data off of a hard drive before reusing it, sending it for repair, or at the drive's end of life. There can be some confusion about what to do when depending on the type of drive. These wipes can take hours or even days as the size of drives increases. Luckily, our drives are much easier to wipe than most.

One of the primary advantages of our hardware encrypted external hard drives compared to other data storage solutions is that the drive can be quickly and easily sanitized via our touch panel interface. Think of AES encryption as a safe with two keyholes. The first key is your password, which is known only by you. The second, which is known only by the encrypted drive, is a 256-bit encryption key. By zeroizing or regenerating the encryption key, the old key is lost. It would be like if you took the key to your safe and threw it into a big pile of keys and grabbed another one.

How many different keys are possible?

The number would be 2^256, as each 1 and 0 is a possible bit. This number translates into:
1.157 × 10^77
This is a huge number. The easiest way to think of it is that guessing it would be like getting your March Madness bracket completely perfect - four times in a row. All the computers in the world guessing keys all day still might take billions of years to guess the proper key. That's why the government and organizations that need to keep data secret use 256-bit encryption keys.

Once those keys are wiped, that data is gone forever.

To perform this wipe on DataLocker devices, you'll need to reset or zeroize the drive.

For more information please read this NIST document detailing more in-depth how erasing the keys securely deletes the data. 
https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-88r1.pdf