In cryptography, CBC refers to Cipher Block Chaining. It is an operation mode for block ciphers.

Normally, block cipher takes a block of plaintext and transforms it into ciphertext in precisely one way. (This is called ECB, Electronic Codebook mode). This mode may be vulnerable because patterns in plaintext are still visible (big blocks of whitespace may look kind of distinctive, for example).

CBC takes the previous encrypted block (or IV if there's no previous block), and XORs it with the next plaintext block, producing a new block. This makes the output look much more random.

This is only one of the many cipher modes that's based on feedback and the reversibility of XOR operation.