Also called secret key cryptography, it relies on a shared secret. The entity that encrypts the plain text and the entity that decrypts the plain text both must know the key.
The two parties can arrange to exchange the key in some different way, sometimes called OOB for "Out of Bounds". For example, I could send you something that I had encrypted via e-mail and then call the recipient on the telephone to relay the value of the key.
Symmetric key cryptography is often much faster than asymmetric or public-key cryptography so it’s preferred for encrypting large amounts of data. But the key length and complexity in current crypto systems don’t make it feasible to transfer the shared secret in a telephone call. So public-key technology is often used to encrypt only the shared secret. First the shared secret is decrypted and then symmetric key cryptography is used to efficiently decrypt the large blocks of data.