Explain AES-256 decrypt.

In theory, you could try to brute force the key and recover the data. In practice, that's extraordinarily unlikely to be successful as it would be expected to take a very, very long time.

Per Wikipedia: AES-256 Decrypt permits the use of 256-bit keys. Breaking a symmetric 256-bit key by brute force requires 2128 times more computational power than a 128-bit key. Fifty supercomputers that could check a billion billion (1018) AES keys per second (if such a device could ever be made) would, in theory, require about 3×1051 years to exhaust the 256-bit key space. Keep in mind, the universe has existed for about 1.38×1010 years. Supposedly, a quantum computer could reduce this substantially, but it would still be expected to take longer than the universe has existed. Of course, you could get really lucky (like win the lottery every week for the next decade lucky) and brute force it in a year or two, but if I were you I wouldn't waste the time or effort.