If you hover the checkmark a tooltip pops up saying that you're subscribed to the thread.

The links change color depending on what theme you're using for the board. I'm using the smaller theme (the one that doesn't stretch to fit the whole window) so for my browser session if I click a link it is no longer bold. If there is a new reply, the link goes back to bold again. If I close my browser and then reopen it and browse back to PT, all links are bold again regardless of new replies or not.

It's just the board's CSS and I really don't think they mean anything now that I've looked into it a little more.

Basically, it means you've already opened/read the thread for that browser session