There are several ways for an instructor to use Moodle to send a message to the entire class:
Course Announcements: Every Moodle course contains a Course Announcements forum. When you post a message here, all the students enrolled in the course will automatically receive an email with the content of that message. Only instructors can post to this forum (using the "Add a New Discussion" button). You can include attachments like videos and PDFs. Both you and your students can easily view past messages that you've posted, by looking through the Course Announcements activity.
Quickmail: Quickmail is an optional add-on which can be turned on for your course. If you would like to use Quickmail, turn editing on for your course, and find the "+add a block" button at the bottom of your left navigation sidebar. Click that button, and you will see a list of available blocks. From the list, choose "Quickmail". The Quickmail block will appear in the right-hand panel of your course page.
There are several ways to communicate with individual students or small groups in your course:
Messages: The Messages tool is found near your name, at the top right corner of the screen. Click on the speech bubble to access this tool. You can adjust the settings of Messages to limit who can contact you via this tool, and you can also manage e-mail notifications here. A record is kept in your Messages of the conversations you have had with individuals. This allows for quick contact between you and individual students. Attachments are not allowed.
Forums for Individuals or Small Groups: You can restrict access to a Forum to an individual learner or a specific group or groups, and use that Forum to provide feedback and otherwise communicate with those students.
Quickmail: Once Quickmail is added to your course, you can send a message to certain students or groups in your course.