Slow learner

Those are rep pts. Lower left under the avatar, there is a scales icon. You can give someone's post a rep pt with a comment - positive feed back or even just hello. You can't really respond to them but you can follow the user's link and comment on their visitor's page.

Instructions to enable your own visitor messages:

Click on User CP on the left side of the blue bar. Then on the left side under Your Control Panel, go down to Edit Options. Click on that, then scroll down until you find Visitor Messaging highlighted in blue. You can enable it there.