Similar to many social platforms, learners and instructors can use the @ symbol plus the name of another member of your Community to call that member's attention to a Post or Comment.
To use this functionality, simply type the @ symbol and begin typing the name of the person whose attention you want to get. A menu will come up with the members of the Community, and that list is dynamically filtered as you type. Click the relevant learner or instructor's name when you see it to select them for the notification. When you submit your Post or Comment, that member of the Community will be notified of your content.
For a broader overview of mentions and hashtags, see Mentions and Hashtags.