Pre-Shipment means that the person shipping the package has printed out a shipping label and getting the package ready to be shipped.
When it's in Pre-Shipment then it means that the USPS doesn't have the package in it's possession yet.
When the USPS tracking information status changes to "Accepted at USPS Origin Facility" then it means that USPS has the package and it's on it's way to you.
Once the package has been accepted they should show an estimated delivery date for your package.
USPS tracking info isn't that reliable and sometimes doesn't update often until the package arrives at your location so no need to worry if it's not updating.