PM or pm stands for private message. A private (not posted to public board) message between you and the person you pm.
Here's how you pm:
Once logged in, you'll go to the wanted ad you want to reply to (click on the topic to open it so you can read the description).
On the right hand side under the username of the person looking there will be some little buttons. Some users have more or less buttons than others but there will always be one there that has a little man and a pm on it.
Click the pm button. That will open a new window where you will enter your reply. It will quote their text and you can either delete that or start your reply below their text. You can choose to preview it so you can see what it will look like to them or simply click submit to send. Either way, you'll want to click submit to send your message.
The message will sit in your "outbox" until the person opens your message.