To mention a user is to draw attention to and/or praise or condemn that user. You mention a user by entering an "@" character followed by the user's user name. The user is notified and the notification (Liferay notification and email) link to the content that mentions the user.
Liferay's Mentions application allows you to mention users in blog entries and message board threads. It's also available to use in message board thread replies and comments--mentions can be used in comments in any app that supports comments.
The Mentions application is bundled with Liferay CE 7.0 and is available as part of the Collaboration Suite.