In the case of symmetric encryption, the encryption key is shared between two parties who use the key on the sending end to encrypt the data, and on the receiving end to decrypt the data. In this case the key is known to the sender and the receiver, who use the key to encode the message at the sender end and decode the message at the receiver end.
As per the symmetric encryption approach, the individual bits and blocks in the message are encrypted one by one, one after the next, e.g., in a text of ‘xyz’ first ‘x’ is encrypted, then ‘y’ and then ‘z’. ROT13 is a symmetric algorithm which “works by shifting all letters in the alphabet thirteen places to the right – A becomes N, B becomes O, Z becomes M, etc. Because there are twenty-six characters in the English alphabet, performing ROT13 a second time results in the original text again – N becomes A, O becomes B, and M becomes Z.” (‘Asymmetric vs. symmetric encryption’)