This setting specifies the number of days a student can be enrolled in this course (starting from the moment they enroll).
If this is set, then students are automatically unenrolled after the specified time has elapsed. This is most useful for rolling courses without a specific start or end time.
If you don't set this then the student will remain in this course until they are manually unenrolled or the clean-up function to remove defunct students takes effect.
If you have selected to manage this course as a meta course, your enrolment period will not be used.