This is a phrase used in baseball to describe a stadium which favors hitters. This is most often the result of a short outfield fence, though in may also have to do with a variety of other factors, including wind or altitude. In order to be considered a hitter's park, the park in question must have a disproportionate number of home runs and other big hits compared to other parks. A hitter's park may be determined subjectively by players, commentators, and fans, or objectively, using a statistic called batting park factor.
The new Citi Field in New York and Coors Field in Colorado are both considered hitter's parks. They both give up home runs more than other fields. In New York, it is a result of a short outfield fence combined with a peculiar wind pattern which is formed by the structure of the seating. In Colorado, the higher altitude results in thinner air, which makes it easier for the ball to travel farther with the same amount of power.