Announcements and Messaging are useful Moodle tools instructors can use to communicate with their students. Announcements should be used to communicate with the entire class. Messages are used to communicate with a single student.
To send an announcement to your entire class:
Note: A notification message will be sent via email to the students 30 minutes after the Post to forum button has been pressed. This gives you some time to go back and edit the message, if necessary.
If you want the message to be sent immediately, check the Send forum post notifications with no editing-time delay box.
For more information, see: https://docs.moodle.org/35/en/Announcements_forum