Think of the boards as bulletin boards. When you post a message it is like pinning it to the top of the bulletin board. New messages push older messages down. Replies to a message push it back to the top. Read: New stuff is always at the top.
You cannot edit your messages, but you have 600 seconds to delete it. If you made a mistake, delete it and repost.
The flag next to each message is for reporting messages to the board moderator.
You can create your own boards. They will remain active as long as new posts are made. If they appear abandoned or are full of spam, the system will delete them.