Cryptography before the advent of computers relied on the use of ciphers. A cipher is a mapping from readable text to gibberish, and back again. For instance, a simple cipher would be to add four to every letter in the text (so A becomes E). Decoding then would involve simply subtracting four from each letter. These processes are called encryption and decryption.
Shifting four places in the alphabet is very obvious, and not secure. More secure is creating a complex mapping that requires a key for translation. Such a key must be distributed to all participating parties. This necessity implies all sorts of logistical weaknesses.