The PM (private message) system on our site works very much like anywhere else - it allows private communication between you and others. Messages can have a single recipient, or multiple if needed.
The easiest way to send a PM to a single person, is to visit their profile page and click on the "Messages" menu, then choose "Send Private Message":
You can also access the PM system directly to see received items (your Inbox), and sent items (your Outbox). The easy way is to use the Quick Links menu (top-left of any white space on the site) which looks like a small triple-bar icon. Hover it, and then choose the "Messages" entry to open the PM page:
Once the PM page opens, it will look similar to the following:
You can switch between your Inbox (received messages) and Outbox (sent messages) with the toggle on the right end.