A USPS network distribution center is a sorting facility where packages and mail go through.
When you have something shipped through USPS the package goes to a local post office and then gets loaded onto another truck and goes to a USPS network distribution center.
The USPS network distribution center then sorts the packages and then places the packages onto another truck or airplane to arrive at another USPS network distribution center.
The USPS network distribution center is a massive building with thousands upon thousands of packages being sorted daily and even through the night.
The USPS network distribution center has conveyor belts and computer systems to scan and sort the packages.
The USPS network distribution centers are always open and operating even when your local post office is closed.