Activity completion allows the lecturer to set completion criteria for a specific activity or resource. The criterion might be viewing, receiving a certain score or a student marking it as complete.
Any activity or resource which has activity completion set will appear in the Module index with a small circle next to it. When the student meets this criteria, the circle will appear filled in.
Activity completion conditions also display in the activity card on the module page and at the top of an opened activity.
In Moodle 3.9, the Module index is not available.