If I get in my private messages that someone has "tagged me" what does that mean? One of the best contributors to my threads has tagged me several times?

 

It means someone posted and tagged (mentioned) your username. The Private Message contains a link to the post where your username was mentioned.

You can disable the notification by going to UserCP

Disable " Receive a Private Message Notification each time I am tagged"

Otherwise, you can read the PM to see the link where your username was mentioned.