To encrypt a message with a Caesar cipher, each letter in the message is changed using a simple rule: shift by three. Each letter is replaced by the letter three letters ahead in the alphabet. A becomes D, B becomes E, and so on. For the last letters, we can think of the alphabet as a circle and "wrap around". W becomes Z, X becomes A, Y becomes B, and Z becomes C. To change a message back, each letter is replaced by the one three before it. However it is very easy to break a Caesar Cipher using Microsoft Excel. Using a more difficult or complicated encryption would be better.
Changing by three is the rule that the well known Julius Caesar used, but the same idea works for any number. ROT13 is a modern version and even easier to break.
The Caesar cipher is a substitution cipher: each letter is replaced by another. No substitution cipher is really safe. The Caesar Cipher is also known as the shift cipher.