Every user can vote for witnesses (aka block producers). The problem tackled here is that lot of those witnesses are almost bound to remain witnesses forever due to a very high staked accounts supporting those witnesses. The vote decay would mean that the longer those high stacked accounts have their witness votes casted, the lesser influence they would have (the weight would be decaying) unless they re-cast the vote thus making the witness spots contestable for other accounts...did I shine at least some light on the topic?:D