Caesar's Cipher is an encryption technique - one of the simplest and oldest one. It replaces each letter of a message by another letter X positions after.
This technique was created by Julius Caesar and used to hide messages about Roma's military strategies.
The technique may have been effective for Caesar's enemies, but can't fool anyone nowadays.
It's pretty simple. If you want to encode a message using Caesar's Cipher, you just need to get each letter and replace it by the letter that stays X positions after. X, in this context, is called the key.
Here is an example for an encryption with key 3:
Stanley is cool <-- original message
Vwdqohb lv frro <-- encrypted message
19 + 3 = 22
^^ ^ ^^
S key V
If you want to decode a message, just do the inverse: subtract the given key from each letter.
22 - 3 = 19
^^ ^ ^^
V key S