Use a Caesar cipher with a shift of 3. Encode HELLO. Decode KHOOR. What does XYZ become?

Move every letter $3$ places along the alphabet.

• HELLO: $H \to K$, $E \to H$, $L \to O$, $L \to O$, $O \to R$. The code is KHOOR.
• To decode KHOOR, move every letter $3$ places back: HELLO.
• XYZ wraps round: $X \to A$, $Y \to B$, $Z \to C$. It becomes ABC.

The % 26 in the program does the wrapping.