When you get enough karma, you can "flag" submissions. If enough people flag a submission, it gets buried and won't show up on the front page anymore.

And just to clarify, if enough people flag submissions, but not enough people, then it can get buried but not get the "flagged" tag? Because very few of the DF posts actually seem to get that tag.

But maybe even some light flagging, plus high engagement-to-view ratio (esp. if engagements go many levels deep fast), may cause some "unwanted" content to be buried.

    And just to clarify, if enough people flag submissions, 
    but not enough people, then it can get buried but not get 
    the "flagged" tag?

As far as I know, this has not been definitively stated by the mods, but it seems very plausible.

As far as I'm concerned it's either that, or explicit moderation action to downweight links to DF and other selected domains.