Some references display in boldface, others don't

I have a library with many references. When I look at the list, some references display in boldface while others do not. What does this mean, if anything?

Read/unread (bold are unread).  You can turn this this feature off in preferences.  

It means nothing but you can bold all references by clicking the bullet at the start of the reference. It is a style only, if you want to keep the bold style you can check the bullet/circle given at the start of the refferneces and if you want to unbold you can uncheck the circle.