It is called a Security Threat Assessment.
TSA will not deal with a motor carrier, per se. This is about Indirect Air Carriers. The carrier or driver is listed as an agent of said IAC, therefore, the TSA wants to know about you. It may have changed slightly in the last year or so. When it started, you needed to have it in order to drop at PASSENGER airlines only. It was not needed for cargo only. That may have changed. If you are a small carrier and want to get TSA certified, contact a customer that books air shipments, such as a freight forwarder. It costs about $30 per driver. Some will pay it for you, because you are representing them. But it is now an annual thing to keep up.
You take a 10 question test that you understand stuff such as the term "IED", and certain kids of activity to be suspicious of.