PM is Private Message.
Example. click on the following link. modified-handbrake-compensator-fitting-t4025-60.html#p59171
Hover your mouse over Arctics avatar on the left of the post. A drop down of available options will appear below the picture. One of which is PM. Click on it, enter your message in the resultant window and send. Your reply will appear in new messages at the top of any page. Alternatively, select new messages then, compose message then, type in the name of the intended recipient in the top field, type in a title and any message then submit.
An email notification is sent to the recipient, as well as notification on the board in case the recipient does not frequent the forum often.