edX courses are either self-paced or instructor-paced. Pacing refers to how course teams run the course, and whether you need to complete course materials on a set schedule.
Many edX courses are instructor-paced. These courses set a fixed schedule. The course team sets specific due dates for assignments and exams, and you complete the course within a defined time period, such as 8 to 12 weeks. In instructor-paced courses, course materials become available at specific times as the course progresses. In most instructor-paced courses, certificates are generated for learners who have passed the course shortly after the end of the course.
In contrast, self-paced courses do not set a fixed schedule. All of the course materials are available as soon as the course starts. You can read more about self-paced courses here: How do self-paced courses work?
How can I tell what kind of course this is?
Check the course introduction page details on the right hand side under Course type. It will read Self paced on your own time for self paced courses or Instructor-led on a course schedule for instructor-paced courses.