A tag is a marker you can assign e.g. to a course. An example is a course that you can assign to last-minute. For this information, you can create a tag:
In this overview, next to the title and alias you can also see, how many courses were assigned to a tag.
If a tag is assigned to a course, the tag will be shown in the course overview (see here).
Tags can also be used to create a menu item, that leads to a course table showing all courses that have a certain tag assigned.