WELCOME TO THE DIS!
Codes are discounts for rooms and/or packages at WDW. If you click on "Discounts" in the menu at the top of the page, you'll link to a page with current discount codes available for the general public, as well as military members, Florida residents and Annual Passholders. The discounts vary from season to season and year to year but one is almost always available, except at the very busiest times of the year. You definitely want to see if one is available for your vacation, because you can save up to 40% off the regular price ("rack rate") of your room.
There's also a sub-board under this Resorts board called Codes & Rates, where the different deals are discussed and others hope and wait for codes to be released for their vacation time. You have to ask for a code to be applied to your reservation, it's not automatic, and it's subject to availability -- only a certain number of rooms are blocked off for use with a particular code at any one time.
As for the acronyms, if you go to the main board, I think there's a thread with the most common ones listed. There are a lot of them and you just kind of pick them up as you go along.
Hope this helps!