It causes their posts to be automatically marked [dead]. Users who have 'showdead' turned on in their profile see these, but other users don't.
[dead] comments that are good can be rescued if enough users vouch for them. To do that, click the 'vouch' link at the top of the comment's page (which you get to by clicking its timestamp). But please only do this for comments that are both civil and substantive.
