(NT) is short for 'no text,' meaning that there is no text in the body of the message. For instance, I may just want to say "Thanks!" That fits in the subject line, so unless I have more to say there won't be anything in the message body. By including (NT) at the beginning of subject title I'm informing all readers of this so that they don't take the time/effort to click on the title and load the full message when there's nothing else to read. The forum software does this automatically if it detects that there is no text in the body, so you don't have to worry about it. You can read about this and more in the forum FAQs.
Hope this helps,