Let's observe the following diagram. There are two devices wanting to exchange a message over an unsecure network such as the internet. Both Bob and Alice are worried that someone in between their machines may be monitoring their traffic and that their privacy is at risk:

With symmetric encryption, Alice would use a secret key to encrypt or scramble her message for Bob. When Bob receives the Ciphertext (encrypted message), he would use the same secret key as Alice to decrypt the Ciphertext to obtain the actual message.

To describe symmetric encryption simply, this method uses the same key (secret) to encrypt and decrypt a message. If the key is lost or stolen, the message is vulnerable or compromised.