The article How Radar Works talks about the basic principles of a radar system. The idea is for the radar antenna to send out a burst of radio energy, which is then reflected back by any object it happens to encounter. The radar antenna measures the time it takes for the reflection to arrive, and with that information can tell how far away the object is.
The metal body of an airplane is very good at reflecting radar signals, and this makes it easy to find and track airplanes with radar equipment.
The goal of stealth technology is to make an airplane invisible to radar. There are two different ways to create invisibility:
  • The airplane can be shaped so that any radar signals it reflects are reflected away from the radar equipment.
  • The airplane can be covered in materials that absorb radar signals.