The "P" stands for Porterville College. There are two other colleges in the Kern Community College District that use the same registration system (Bakersfield and Cerro Coso), so the first letter of each college is used to indicate on which campus that class is offered that you registered into.
Each academic discipline offers a variety of courses within that discipline. The course numbers differentiate the courses within the discipline. For example, History 117 is American History before 1877 and History 118 is American History after 1877.
Courses that have a number of 100 or more are transferable.