What does Origin Post is Preparing Shipment mean?

It means that the item is already in the country to which it was sent to. So, if you had ordered something from Canada, and you live in the United States, it means that the item is already in the United States and is preparing to be shipped by the USPS. Origin Post is a company that takes the items from overseas and sorts them.

EDIT: Origin Post is not a company. Origin post = place where it's being shipped from's post office/system. So, it's not in the US.