Resetting your course can be useful between lots of students to purge out old user data so that the course is fresh. Of course there are times when you wish to keep old user data (like previous forum posts for students to build on) so be careful using this feature.
You'll be taken to a settingspage where you can choose what you wish to reset in the course.
If you're using weekly format (rather than topicsformat) this can be a good place to change the course start date so that the weeks display properly for the new semester. Be careful deleting anything else in this section unless you're really sure.
Then, depending on which activities you have in your course, you may see some other settings areas:
These allow you to delete the data from any of these activities. Your activities themselves remain, ready to be used by the next lot of students.
Note that you will still need to go through assignments and other items that have due dates set to make sure that the due dates are updated for the new lot of students!