This is a baseball term used to refer to a stadium in which home runs are difficult to hit. A pitcher's park will usually have a back wall very high or very far back compared to other parks. For example, in Boston, Fenway Park has a large green wall in left field that is hard to hit over. It has earned the nickname, "Big Green Monster." A pitcher's park may also have odd shaped walls or other factors that make it difficult to get a ball out of the park.
Although there are many unique factors on every field, Major League Baseball has made it difficult to have a pitcher's park due to the rules and restrictions which govern the size and shape of the field. The goal of the new rules was to prevent teams from having too large a home field advantage by abusing or pushing the boundaries of prior looser restrictions. Furthermore, the modern game of baseball is geared towards offensive production, which has led to rule changes that favor hitters rather than pitchers.