Tags should be assigned to courses, so users can filter for those tags when viewing courses in the Registration portal.
Tags are created in the Configuration Lookup Lists (System > Settings & Security > Configuration):
Configure tag lookup list
Once a tag is created, it can be assigned to a course.
Link to Courses
In the edit mode of the course, select the tags which apply to that course.
Once the tags have been assigned, when users access the Registration portal, they will be able to filter for courses based on one or more tags.