Symmetric encryption is a type of encryption where only one key (a secret key) is used to both encrypt and decrypt electronic information. The entities communicating via symmetric encryption must exchange the key so that it can be used in the decryption process
A Symmetric-Key Algorithm uses a string of data to encrypt and decrypt information. This string of data acts like a real-world key which can lock and unlock a door. In fact, it is often called a "key" or a "password". With symmetric-key algorithms, the same key is used for encrypting and for decrypting (that is what makes it "symmetric").
Anyone who knows or possesses the key can decrypt the information. Anyone who does not know or possess the key cannot decrypt it easily. It cannot be simply unscrambled like a substitution cipher. Someone would need to use advanced techniques to "break" the encryption.
If the algorithm is hard to break, this is a better way to encrypt messages than a substitution cipher. Messages can be sent publicly, but only the recipient who knows the password can decrypt it. Much of the history of cryptography (and military communication/intelligence) has been dedicated towards either developing stronger algorithms or trying to break current algorithms.
The rapid increase of computing power beginning in the 1970s transformed the cryptography landscape. Hundreds of algorithms have been developed and hundreds have been broken. There are three algorithms which are notable for their resistance to decryption and their wide-spread usage.