A mention is a reference to a member, a group, an application, or content, within content. Mentions are added through the content editor and are shown as links to the member or location mentioned. When a mention is added, the member or owner of the location can be notified (based on their notification settings) that they were mentioned to call attention to the content and improve targeting of responses to otherwise wider-targeted content.
See How do I use the content editor? for details about adding mentions to content.