how does Network Theme decide what posts to display on Network Homepage?

We can't figure out when/why certain posts are pulled/not pulled into the feed of posts on our network homepage (www.first2board.com) It also seems to show some very old posts and not the most recent post that was written and published.

Is there any documentation that I can read or can you point me to where this is being set so we can adjust it.

Thanks
Stacey