Radar works by sending radio waves out from an antenna and collecting the waves that bounce back from any objects (see How Radar Works). On a radar screen in an aircraft or in a ground-based radar station, an airplane appears as a blip. The larger the aircraft, the larger the blip appears on the screen. Other objects, such as flocks of birds, can also show up. Aircraft designers have worked for years to minimize the radar signature of an aircraft. If the radio waves are deflected or absorbed so they don't return to the radar antenna, then the airplane is invisible or could be mistaken for a flock of birds or other non-threatening object.
Aircraft designers use irregular sawtooth edges, a jumble of curved surfaces and other design tricks to deflect radar waves in unexpected patterns. Planes are painted with thick paint that can absorb radar waves instead of reflect them. The idea is to make the aircraft appear to disappear into thin air.