Discount codes are Disney's marketing tool when they are looking to increase attendance usually during non-peak periods. It's a 3 or 4 letter code that you might see advertised in a local paper or stumble upon on this website or mousesavers.com. Sometimes the codes apply to everyone and sometimes they could be for residents of a particular state. If you have an annual pass, there are hotel discount codes for this as well. There is no rhyme or reason as to when codes are released.
When you find one, yes you call CRO and ask them to apply the code to your reservation and then re-calculate the cost.
Hope this helps!