An envelope will show up by the thread when you sign up to have updates to a discussion delivered to you by email. If you click on "your digests" you will see all of the discussion updates you've signed up for and you can unsubscribe to those digests by checking the box beside the thread and clicking on the "unsubscribe" button.
If you mean the little red flag, it's there because it's a post or thread that you have contributed to.
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