A prerequisite is a course you must have completed before registering for another. (A corequisites is a course you are taking at the same time or prior to registering for another). All prerequisites are listed in the course description for each course. Prerequisites are in place because for some courses prior knowledge of a topic is requiremed.
If you don’t meet prerequisites (or corequisites), you will be dropped from the course by the department offering the course.
Some departments list suggested courses to take prior to registering for another. These courses are not mandatory, but the department feels the courses help the student better prepare for the course material. Students could consult their Dean or Major Advisor before taking a course without the necessary suggested preparation courses.
If a course lists "permission of the instructor" in the prerequisites, students may seek the permission of the instuctor of the course to register. These students will need to get a permission number in order to register.
If a course does not list "with permission of the instructor" with the prerequisites, then if you do not have the prerequisites you cannot take the course.
No, not all courses have prerequisites. If there is nothing listed in the course description, then the course is open to all students.