When we talk about codes, we are talking about discount codes for Disney resort hotels. They are all released by Disney and are marketed to different demographics of people. For instance, they may place an ad in a newspaper, and say to reference code ABC (or whatever) when making your reservation. This is so the Disney phone representative will be able to pull up the discounted rate on their computer.
A good website to stay up-to-date on what the latest codes are is mousesavers.com.