PM means "Private Message". Welcome to the board! If you click on someone's user name, like where it shows up in blue on each person's post, you will get a box of options, and one of those options is "send a private message". Click on that and you will get another box to type in a message to that person, click "send" at the bottom. Only they will get your message. When they answer you, you will see it in the black "Notifications" at the top right of your page when you log in. You click there to retrieve and read their message back to you. You can also respond to them again from there.