Certificates are automatically generated upon course completion to signify that a user has successfully passed a course.
Navigate to the course that you want to add a Certificate for. In the General Settings tab, check the box that is labelled Course Certificate, then use the drop-down menu to choose either the Default Certificate or a custom certificate. Once finished, hit Update Course Settings at the bottom of the page.
Here is what a certificate might look like.
You can also add custom certificates. Too find out how, please see this article.