It means that it was registered in the system in the post office in Brooklyn, NY. The departure means that it has left NY and is on its way to its destination. You should have your package in about 3 days, so probably by Monday.
United States Postal Service
Seriously? It means United States Postal Service. How is it possible that you don't know that?
United States Postal Service --when you go send something at American post offices, it is sent via USPS. UPS (the brown truck) is something's entirely different.
It means the item got to Brooklyn yesterday morning and then got sent on its way from Brooklyn today at some point.
It will get there when it gets there. Knowing what city it's in at every step along the way isn't going to make it arrive one second sooner than it would otherwise.