I always looked for a simple definition of the concepts of cryptography until I heard someone saying: “Cryptography is the art of keeping everything public except the keys”. And it’s incredibly succinct.

Generally, if person “A” wants to send secret data to person “B”, it will be done through the network. None of A or B has control of the network or the internet. So they can’t forbid a person “C” to see what they are exchanging on the net. So they should do it in a way that Person C (generally called men in the middle) doesn’t understand anything while reading the content. Here comes cryptography.