Static test airframes, or more commonly called, “iron birds” are partially built, non-flying airframes or old formerly flying airframes that are used by agencies and manufacturers to test either the strength of than airframe, various design components or aircraft subsystems (avionics, flight control, engines, etc).
The iron birds used for strength testing are typically full-scale representations of the aircraft that are rigged to giant gantry cranes with weights and strain gauges attached. See the pic:
Once installed on the cranes the airframe is literally pulled and pushed to properly simulate all the aerodynamic forces that the aircraft will encounter throughout it’s flying career. Often the iron birds are tested till destruction.
Some iron birds are formerly flying airframes that have accumulated too many flying hours and are no longer consider safe to fly. These aircraft are typically stripped of most equipment (engines mostly) and used to test various aircraft subsystems in support of other programs.
Iron birds aren’t limited to NASA. The US military also used them for the same purposes.
You can learn more about that particular aircraft here.
As an aside, old airframes are also typically used as maintainance trainers in the military. These are called ground instructional airframes: