You’ll see a little blue dot with a number inside beside a thread. That tells you how many new posts that you haven’t read are in that particular thread. Hover your mouse over it and you’ll see.
A blue dot next to a thread means it’s a brand new topic. If you hover your mouse over it you’ll notice there’s a message.
The numbers beside each subject on the main board are the # of new threads posted for that week. Hover your mouse over this and you will see the message.