Colossus computer
The coding machine added together the plaintext (the un-coded version of the message) and a stream of characters (letters, numbers, punctuation) called the keystream (a stream of seemingly random characters) that it generated to make the ciphertext (the coded version of the message). The ciphertext, which made no sense, was transmitted by radio. At the receiving end an identical machine removed the keystream to produce the plaintext of the message.