When a package is going through USPS and the tracking on USPS says item is currently in transit to the destination it means that the packages is traveling either by airplane or on a truck being taken to the next USPS sorting facility or to your local post office.
If the package is in transit to your local post office then it will usually arrive within a few days after the tracking says in transit to the destination.
So it basically means that the package left the sorting facility and is traveling either down the road in a semi truck trailer or by airplane.
The tracking info can stay stuck on the status "In Transit To The Destination" for a few days to 4 days depending on how far it has to travel.