Now that I re-read your OP, Codacap, I understand you're asking about the difference between bolded titles and unbolded titles. A title appears in bold if it has not been opened on your computer, or if a new post has been added to the thread since you last opened it. Once you open a thread, the software
"remembers" that you did so and changes the title to unbolded. This is done to help you recognize at a glance if a post has been added to a thread since you last opened it - if the title once again appears in bold type, a new post has been added since you last opened the thread.
Sorry for the misleading stuff I posted, above, about changing thread titles.