A one-time pad is a form of encryption that cannot be cracked, so decryption is only possible with the key. It was first professionally described in 1882 by Frank Miller. Despite being uncrackable, it has several failings that make it unattractive for modern use.
Using a one-time pad, you must have a key that is at least as long as the plaintext. To encrypt the plaintext, you process each bit of the plaintext with each bit of the key and perform a reversible calculation like modular addition. The result is the ciphertext. For example:
Assuming A=1, B=2, C=3, ... Z=26, and space=0. plaintext: ATTACK TONIGHT key: IQENEPLRB ZAZF ciphertext: