What's In Transit means in USPS?

If a shipment is “in-transit” then that means that carrier has scanned the package upon pickup and it is now en route to the end customer.
If a shipment stays ‘in transit’ for a prolonged period of time, it is possible it may have been delayed. There are many things that can delay an order past its original expected delivery date - poor road conditions, weather, and deliverability of the location are a few examples.