Using that 'Mark Therad Resolved', one can close a thread by himself after he got a perfect answer to his question posted on the thread. so no body will digg that thread again.
But now i feel, that here there is no question and answer type of systems, it might not requir.
For just to see an example, visit vbforums.com , goto a thread, and click 'Thread Tools' and you will see this option. Further,
Threads that are marked with a Right sign are threads that are answerd and closed (closed mean not locked for posting).