Sometimes bold, sometimes not - why??

When I look at my computer, sometimes the threads are bold and sometimes not (just regular type.) Why the difference??

 

when you post you can make it BOLDor not. Not quite sure why you would want too.

 

The ones in bold are the ones that changed since you last made an action. So if you log in, read a bunch of stuff, then log out and log in later, the bold ones are the ones that changed since you logged off or posted last.

 

Thanks, Squablow, now I got it.