We can mention other members in our comments, drawing their attention to things that we think they will like, or be interested in, or engage in the conversation.
You do this by typing "@[persons name]' into a comment. A list of members' usernames automatically appears and narrows as you type. Then you choose their name and hey presto, they are mentioned.
These will show up in those members' notifications and in their email alerts if they have set their account settings to send them emails.