In your Moodle course click Participants and then from the drop-down menu Enrolment methods.

 

 

From the Add method drop down menu select Guest Access. 

It is possible to add a password for the Guest access, but it is not necessary. Click Add method and the guest access is ready to use. 

Teacher can give the course address as a link to the Guest. After clicking the link the Guest has to use "Log in as a guest" on the Moodle login page.