In all other forums I’ve used the number shown within parenthesis in the page title shows how many notifications and new private messages you have. Here it shows new unread topics.
This number shown is quite arbitrary since it only shows the number of topics that has been created since you loaded the page, not how many unread topics you have. Also I find it quite stressful to see that number. I always think someone has written something to me and hurry back into the tab to see what it is, only to discover that it wasn’t for me. This actually often makes me close the Sketchup Forum tab as soon as I’m not actively browsing the forum, when I could instead have it constantly open.