In transit on USPS tracking just means that your package is on the truck and on it's way to either your local post office or to a sorting facility.
When the USPS tracking says it's in transit it doesn't mean that it's gonna be delivered today but just means it's on it's way to you.
Once the tracking says that it's arrived at your local post office then the package will likely be delivered on that day.
When USPS tracking information says in transit to destination it means that your package has been processed through a USPS sorting facility.
After that the package has been loaded onto a truck or plane and is in transit either down the road or in the air and on it's way to it's destination which is your location.
It doesn't always mean though that the package is gonna be at your post office the next day but it just means it's on the way to your location.
Sometimes your package may arrive a few days before the expected delivery date or sometimes it's delivered on the date it's expected.
The tracking information won't update for likely a few hours to a day or so because the package has to be taken off the truck or plane and go through the sorting facility again to be scanned.