Guest access has been enabled for you to view some courses. Select a department then a course you wish to view. You will be brought to a login page and from here you just select the "Log in as Guest" option. There are some courses that do not allow guest access due to having quizzes.
StudentsClass of 2019: Your accounts have been setup within Moodle.
Your credentials will be the same that you use to access your PowerSchool account except that the username is lowercase.