Caesarian Shift Cipher Applet

Lane Tait
The applet below is a representation of a Shift Cipher that allows one to cipher and decipher messages through the method of what is called a "Caesarian Shift."
What does it mean when Shift=0? 5? 23? What would the message "I love mathematics" look like when using the Caesarian Shift Cipher with a Shift at 7? I order to decipher an encoded message, we need to set the Shift so that is set at 26-(the original shift value). So if I encoded a message at a Shift of 14, what Shift setting is needed for someone to decipher my message? I send you the message "DRICVP REU DV" where I encoded the message at a shift of 17. What was my original message?