A symmetric key is a type of encryption method where the same key is used for both encrypting and decrypting information. This means that both the sender and the receiver must have access to the same secret key to communicate securely. If someone else obtains this key, they can easily read the encrypted messages. This method is efficient and fast, making it suitable for encrypting large amounts of data. However, the main challenge is securely sharing the key between parties, as anyone with access to the key can decrypt the information. Common algorithms that use symmetric keys include AES and DES.