Hi Truck,
A "PM" is a 'private message'. This is a medium that allows you and others to contact other IIS members on matters between yourselves. It's like an internal email system.
If you look in the top right hand corner of an IIS page, you will see your username in blue, along with the heading 'Private Messages'. Click on this heading and the relevant page will come up. Its defalt page is the 'received messages'.
I'll leave the rest for you to explore there,

.