People have been asking what this is, so here you go.
Basically once a thread is posted it is 'live', or if multiple people are posting to an existing thread it can become 'live' again, that means people who are viewing it don't have to refresh in order to see new posts. Sit and watch a busy thread for a few minutes and you'll see what I mean. Posts will seem to appear out of thin air.
Here's an example: