The above video is a quick and simple explanation of a simple one time pad by Khan Academy. The video shows how one-time pads are used to create unbreakable messages. The one-time pad exhibits perfect secrecy, but at the cost of a key the same size as the message. The key must never be reused and must be destroyed by both the sender and receiver once the message has been decrypted.
Here is a more in depth explanation from the CompTIA Security+ course.
And another explanation from numbers-stations.com.