What is the significance of whether a title is bold or not bold?

 

Never mind, I got it. If a thread has a new reply, and you haven't read it since the new reply was added, the title goes bold. What was confusing me about it, if you use the "back" button on your browser, you get the old page, so the title is still bold. If you click "refresh" the title goes to not bolded.

 

you are correct. It is basically it is any thread you have not viewed since either since your last visit or since a reply was added.