yea, that's the question.

It's an another crew that is normally directed by someone other than the main director. They often do establishing shots or stuff not involving the main actors or in another remote location. It's meant to save time and money by passing off less important shots to another, normally smaller crew. 2nd ADs sometimes are 2nd unit directors. If the shot is stunt intensive, the stunt coordinator can be the 2nd unit director. Or, the director or producer will hire an outside director for the 2nd unit.