College preparation is the class you will take when you are not being challenged by the honors or AP version of a class, and when you are not placed in the corrective version of the class to catch up. One example of greater access to AP courses in Washington is computer science. Ninety-five Washington schools offer AP Principles of Computer Science courses; 32 of those courses began this school year. The purpose of college preparation classes is to prepare students for the most rigorous and challenging workload they will have to deal with in their first year at a university.
Advanced Placement courses are popular in public schools, but are becoming extinct in preparatory schools for private universities. If your high school offers both college prep and honors classes, chances are that CP courses are meant to help you with point 1 above. Some courses have been selected only by high school as honors, but have not been approved by universities as honors courses.