A well$-$known example of a substitution cipher is the Caesar cipher, which is a substitution alphabet. A popular example of this is referred to as R O T $13$ or ROT$-13,$ where the alphabet is rotated $13$ places, but really ROT$-13$ is a Caesar cipher that uses a key of $13.$ Using our hello world example and walk through encoding it using our ROT$-13$ cipher, our ciphertexts winds up being URYYB JBEYQ. To reverse this process and go back to the plaintext, we just performed the reverse operation by looking up the characters in the output side of the mapping table. You might notice something about the ROT$-13$ mapping table or the fact that we're offsetting the alphabet by $13$ characters. Thirteen is exactly half of the alphabet. This results in the ROT$-13$ cipher being an inverse of itself.

Please explain this to me in simple language so I can understand.