An @mention is like a gentle tap on your shoulder a way to get your attention in a channel conversation or chat. 

To @mention someone, just type @ in the compose box and select their name from the menu that appears. 

When someone @mentions you, you get a notification in your Activity feed, and you also see a number next to the channel or chat you were @mentioned in. 

Click the notification to go directly to the point in the conversation where they mentioned you.