It means there is at least 1 out of date interpart link (e.g. WAVE linked body), interpart expression or assembly constraint within the structure of your displayed part.
What are the reasons for this state?
In my opinion there may be two things: You have delay interpart expressions or delay interpart geometry enabled. This is the reason for a interpart object still being out of date. The other thing is you don´t have the source part available. I just had this case today. A designer was working on a part and did update some interpart geometry links. He did not update the interpart expressions which were still out of date. The CAM programmer copied the very same part to his environment. Without access to the skeleton parts driving that parts interpart expression, he was not able to update the link.
How can you get rid of it?
If everything that is needed for the update is available and loaded just call the Update all command. If the source part of a interpart link is missing, you have to live with the (!) or break the link.
I am quite sure this is complete, although I could not access the NX online help to check. It is not working anymore on an iPad.