When a USPS shipment is pending it means that the USPS has accepted the package at the post office but they have not inspected it for proper postage etc.
Once the USPS scans the package and checks it for proper postage then the package will be accepted and then moved onto a sorting facility.
Then from the sorting facility the package goes onto other trucks, planes etc to be delivered to your local post office.
The USPS packages go through several sorting facilities if the package is moving through different states before it gets to you.
Most packages will be accepted though as long as the package has the proper postage.
It can take up to 24 hours to 2 days for the USPS tracking to update and sometimes the packages do not get scanned all the time although most packages in sorting facilities get automatically scanned when they are being sorted.