When the USPS tracking information says that your package is moving within the USPS network it means that the package is moving through their network.
That could mean that the package is either on a plane, semi truck etc and is traveling from one USPS sorting facility to another sorting facility that gets it closer to your local post office.
It can also mean that it's being sorted in the sorting facility as well.
The information that USPS gives on their tracking information can mean different things but when the USPS tracking says in transit then it means that it's on a truck or plane and on it's way to your local post office.
When the package does get to your local post office then it will update to say that it arrived at your local post office and then out for delivery or delivered when it is delivered to you.