It stands for the National Poultry Improvement Plan. If you don't know what it is, you are most likely not certified.
They test your flock for pollorum and salmonella. You can also be tested for other diseases and be certified disease free but usually it costs extra to do it and you have to test your entire flock every quarter for the other diseases.
The other diseases they can certify you for are MS, MG,
MM and AI.