See the code in its entirety here.
This code exploits a "padding oracle" in order to decrypt a ciphertext encrypted with AES-128 in CBC mode.
The oracle decrypts a ciphertext and then determines if the resulting plaintext has valid PKCS#7 padding. This may seem innocent, but because an attacker can provide arbitrary text to the oracle, it can actually provide sufficient information to decrypt a ciphertext without knowledge of the key.
Each block of ciphertext is first decrypted with the key, then XORed with the preceding block of ciphertext (except the first block, which is XORed with the initialization vector) to produce the corresponding plaintext block.