When adding an activity to your course, you can specify whether it is always available to the learner, or whether it is only available under certain circumstances.
For example, you might make availability dependent on the learner having completed a previous activity thereby ‘releasing’ material in a controlled fashion as a learner moves through the course.
Restricting access is a mandatory setting for the course certificate! Here, you must choose one or more activities or the whole course to be completed. The reason for this is that the learners in your course should only receive their certificate at the very end after having done all necessary activities to pass it.
Restriction can be based upon “Activity completion”, “Course completion”, “Date”, “Grade”, the “group” or grouping the students are in, or even user profile fields. The “Restriction set” button also allows for more complex criteria requiring nested conditions.
If you want to restrict access to single activities, e.g., to make sure that your learners have completed a previous activity first, you should choose “Activity completion” as condition and select the activity that needs to be completed first.
For the course certificate, the restriction “Course completion” makes most sense. This means that the learner needs to complete all activities in the course first before they will receive the certificate. Please note that this only works if course completion settings are configured as well.
After you have set up your access restrictions, click "save and display" and go back to your course. Your learners will now see a padlock icon displayed next to the item in your course informing them about the conditions under which the lock will be removed:
As always, there is also a broad documentation on access restriction available on Moodle Docs.