Markus Jayne explains how dominant, co-dominant, and recessive morphs work pretty well (remember dominant and recessive alleles or traits in high school biology? )

http://ballpython.ca/genetics.html

Have a look at the illustrations and see if it helps you understand. A lot of popular morph bases are codom, such as spider or pastel, meaning they only need to get the pastel (or other co-dom/dom) allele from 1 parent to "show" as a pastel, whereas recessive needs the allele from both parents to show. I'm sure other places on the internet can explain it much better!