In symmetric key encryption, the sender encrypts the message using a secret key and sends the encrypted message to the recipient. The recipient then decrypts the message using the same key. This process ensures that even if the message is intercepted during transmission, it cannot be read without the key. For example, if a business wants to send a promotional offer to a customer, they can use a symmetric key to encrypt the offer details. The customer, who has the same key, can decrypt the message and view the offer. This keeps the promotional information secure from potential eavesdroppers.