Instrument Flight Rules, or IFR, is a rating attained by pilots who need to fly through thick cloud cover, in low visibility, or inclement weather. The pilots who gain this certification must be able to fly the aircraft solely by their flight instruments and radio navigation, because they are not able to see out of the cockpit window due to low visibility conditions.
A pilot must be cleared by the air traffic controller to begin flying the aircraft by IFR, and the distance between separate aircraft is closely monitored by a ground controller through radar readings. A flight plan is required to be filed with the appropriate authorities to gain clearance to perform this type of flight. A clearance will usually have a limit to how far the plane can fly before a new clearance is required, and the pilot is to maintain a strict heading and elevation while flying the aircraft by IFR.
In order to fly IFR in the United States, a pilot must have an instrument rating, meaning he is extremely familiar with all of the precise instruments onboard an aircraft, and must be current on IFR flight, usually meaning he or she has completed several instrument landing approaches, is familiar with radar systems and tracking, and has flown holding patterns within the last six months.
An aircraft must be equipped and type-rated with all of the mandatory instruments in order for them to be flown by IFR. The instruments must also have been inspected within a certain amount of time to qualify for IFR flight.