An announcement is the text that students see at the top of the course's main page and every lesson page. It's the only one for the entire course.
To post an announcement in a course, open the Courses section, select a course, and go to the "Announcement" section in the learning settings. Enter the text (formatting is optional) and click "Apply." As long as the text is filled in, the announcement will be visible to students. Clearing the text will hide the announcement.
These are settings for the course's learning process, not settings for the digital product.