The most widely used symmetric encryption algorithm in the world is the Advanced Encryption Standard - AES.
The concept is simple: two parties share a private key, often referred to as a pre-shared key, which is then used by one side of the communication to encryption the information they want to send. The other side of the communication uses the same key to decrypt the information so that they can consume it.
This looks flow something like this:
Anyone listening in the middle (known as a "man in the middle attack") will be unable to decipher/decrypt the information without the pre-shared secret key.
Symmetric encryption algorithms are very secure, and very fast. But there's a problem: how do you get a pre-shared key to another person, so that you can send secure messages to each other, without someone intercepting the message containing the secret?