This tool will help you solve challenges related to shift ciphers, which are also commonly referred to as a Caesar cipher or a ROT cipher. These are often included as easy cryptography challenges in a CTF.
Shift ciphers are one of the oldest and simplest methods of cryptography. The way they operate is as follows: Pick a number between 0 and the number of characters in the alphabet you use. This number will be your "key" for the encryption. To make the ciphertext you then simply "shift" all characters in the plaintext by the number you defined as the key.
To decrypt the cipher-text you tell the recipient the key you used, and then they will be able to decode it following the same approach. Just "shift" in the opposite direction compared to the way it was encrypted.
With current technology, then it is trivial to break this encryption, as you can see using the tool on this page. Calculating all possible results with any possible key does not take much time for a computer to do, and if you are determined enough then it is possible to break the encryption by hand.