The state of a hockey or soccer goal when its designated guardian, the goaltender, has gone for an ill-advised jaunt about the playing surface or is otherwise caught out of position.
Also, the title of a book written by George Plimpton describing his experiences with the Boston Bruins including his bried ordeal in net during an exhibition game. Fans of the game of hockey, and especially those 70s Bruin teams willl enjoy it.
As a metaphor it represents a situation where it should be child's play to accomplish your goal. Primarily used in the negative sense, after someone's blown an easy chance: "Aw, man, you had the open net!"