When the dot’s yellow, does that mean someone is on-line or that there is a new thread? Stupido question, but I’m asking nonetheless. 
When you see the “CC” square below someone’s post count is lit up (yellow), it means that person is online.
When the dot before a thread is lit up, it means there are new replies to that thread or you haven’t yet read that thread before.
Thank you, mercurysquad!