The @mention functionality allows you to link a person, place, or piece of content in the Community.
When you @mention a person the user is sent a social interaction notification in their Inbox. Depending on their preferences they will also get an email notification. When you @mention a place it will be noted in the places activity stream.
Whether you're posting a status update, asking a question, posting a comment or creating a document in the community, simply type the @ symbol and begin your reference. A search dialog with suggestions will be listed.
For example, if you wanted to reference this guide, you would type @How_do_I to begin referencing the list of all content that start with "How do I..." Keep typing to narrow the choices further.