PM means Private Message.
Click on "Send message" to the left of the post text and you will get a "message form" where you can type your message.
This message will only go to the member who made the posting, it cannot be seen be anyone else including the administrator (me). It is totally private and only the person it is addressed to can see it plus there is a copy for the sender to see. To see your messages go to "My profile" (Top of screen)and click on "messages". The system keeps a copy of all messages you have sent and recieved from other members and nobody can access them.
The technical stuff (simplyfied because I am not a teccy): when you send a PM is is kept on the system, it does not go outside of the system. An email is sent to the recipient telling them that there is a new PM. You can turn this notification off in which case you will have to go into your profile to see if you have any PMs. If you get a notification simply click on the link and it will take your to your PM - you will have to log in to read it. You can delete any message you no longer want but there is no limit to the amount of messages you can keep.
Hope this helps,
Caps