The ground rules are the rules specific to a particular stadium or field of play. They cover such issues as when the ball is in play or out of play depending on the specific outline of the grounds, what is the exact delimitation of the outfield fence, and how to deal with specific obstacles. For example, it is the ground rules that determine what happens if a ball hit in Chicago, IL's Wrigley Field gets caught in the ivy covering the outfield fence. The ground rules also deal with objects protruding over the field of play, such as loudspeakers.
The most common use of the expression is in the phrase ground rule double, which refers to a ball that bounces out of the field of play over the outfield fence. However, this is not technically a ground rule, as it applies throughout baseball, regardless of the field of play.