An Instrument Rating is a rating that is added to a Private Pilot or Commercial Pilot License and refers to the qualifications that a pilot must have in order to fly under Instrument Flight Rules or IFR.
It requires obtaining additional flight training from a qualified Certified Instrument Flight Instructor (CFII), including rules and IFR procedures to enable the pilot to legally and safely operate a properly equipped aircraft in Instrument Meteorological Conditions(IMC). Additional instruction in weather and more intensive instrument flight training in flight solely by reference to instruments is the core of Instrument Flight Training.
Testing consists of a knowledge test and a practical test (known more commonly as a check-ride). The check ride is divided into an oral part and a flight part. For most pilots, the most significant value of flying under IFR is the ability to fly in instrument meteorological conditions (such as inside clouds). Additionally, all flights operating in Class A airspace must be conducted under IFR.
In the United States, thus any pilot who plans to operate an aircraft capable of flying above 18,000 feet, such as a turbo-prop or a very light jet (VLJ) will definitely need to have an instrument rating. An instrument rating is also required when operating under Special Visual Flight Rules (SVFR) at night.