The '@' symbol is a special character that will direct a message to a user and let them know you mentioned them in a post.
After typing '@' you essentially open up a search for any user searching username, first name and last name, up to three words in the search. When you select from the list, it subs in the username, which is the unique identifier that the notification system needs.
For example, to notify Sean Gaulin (Support Lead for Linux Academy), with username SeanG, about your post, just include '@SeanG' in your post. If you don't remember the username, typing in '@Sean Gaulin' and clicking on the name in the search will make it @SeanG in the post.