On USPS tracking when the USPS says the item is currently in transit to the destination it means that the package is on a truck, plane etc and is traveling to the destination.
So before the USPS tracking updates to say the item is currently in transit to the destination it should say that your package has left the sorting facility.
And after the package leaves the sorting facility it's put on a truck, plane etc to be delivered to the next sorting facility closer to your address.
Your package can go through a few sorting facilities before the package actually arrives at your local post office to be delivered to you.
Basically when the USPS says the item is in transit it means that the package is traveling down the road or in the air on a plane to reach it's destination.