When a course reaches its maximum number of registered students, you will see an option on Section Search to add yourself to the wait list. If someone drops the class before the semester begins, the first person on the wait list will be notified. If a spot has opened up for you, you will receive an email with instructions to register for the course if you still want to take it. You will only have 24 hours to register before losing your place, so you will want to get that done quickly.
Please remember that putting your name on the wait list does not guarantee that you will get into the course. Seats will only become available when other students drop.