Canvas offers two main methods to communicate with students:

• Announcements: Whole-class (or whole-section) mass communication.
• Inbox: Messaging to one, multiple, or all students in a class. 

Announcements

Announcements are the preferred way of contacting your class as a whole. An announcement will both post to the announcements area of your class and send your students a notification with the contents of the announcement.

Canvas support document for announcements: https://community.canvaslms.com/t5/Instructor-Guide/How-do-I-add-an-announcement-in-a-course/ta-p/1194

Note: Announcements will not generate an email if your course is not published or the course start date has not happened. You can check the course start date in the course settings and you should publish your course before posting an announcement.

Inbox

Inbox is a self-contained messaging system within Canvas. It's not directly affiliated with your course, though there are filters that allow you to select your course participants. Inbox should be used sparingly, or in situations where you only need to contact a specific student rather than the whole class.

Canvas support document for inbox: https://community.canvaslms.com/t5/Canvas-Basics-Guide/What-is-the-Inbox/ta-p/55