Usually it means something like this, but in the case of the challenges the expression is (mis)used to refer to giving other entrants a low mark (typically half a star) to reduce their score and hence increase your own chance of wining. It is not uncommon for the winners (and other highly placed entrants) to have several half stars when the rest of their scores have a very high average.
From a combination of observing the distributions of ratings, my own judgment of the photos, and what the challenge host says (about eg one voter giving all but one of the photos a half star), I am pretty confident that these half stars are people trying to reduce the chances of photos they think will score well.
Interesting.
