Lounge suit is a loose term for women, but it generally means that a formal dress or outfit is required. However, it isn't quite as formal as black tie, so something like a pantsuit or floral pattern could be acceptable. For men, it's a lot simpler and means a suit worn with a tie.
Mostly, yes! And in most cases, you can leave the airport during a layover in the US, even while traveling to an international destination. International travelers need to make sure they have a valid visa to travel in the US. But, whether you need to leave the airport or not during a layover is completely up to you.