It means the same as when I explained it to you last week, in a previous post in this ad.
PM means Private Message/ Private Mail on most forums, and that is what it used to be called on this forum until the software got upgraded. PM SENT means someone sent you a Private Message, check you in box.
Now on this forum the button for it is called a "Conversation", and I sent you one. But most folks still use the short hand of PM.
Look at the upper right corner of your screen when you read this post, you should see in the blue bar up there, an envelope icon with the word Inbox next to it.
If there is a bright read square over the envelope with a number in the red square, that tells you that you have a PM or Conversation waiting for you to read.
Just click on the envelope to open your inbox, then click on the message (s) to read them. The number in the red square tells you how many messages (conversations) are waiting, unread, in your inbox.
Steve