The codes unlock dialogues from a previous visual novel and an alternative secret route.
In the downloadable extras, there is an image with all the codes.
But in case you just want to know what they are without further ado:
*POSSIBLE SPOILERS*
- Rebeca ~ (The protagonist of DR).
- Daniel ~ (The second protagonist of DR).
- 1125 ~ (Answer to the riddle).
- Miroku ~ (Secondary character in the first story).