Radar is an object-detection system that uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraftguided missiles,motor vehiclesweather formations, and terrain.

It stands for 
Radio Detection And Ranging

but I am not understanding
ok.. it sends the radio waves in a particular direction. it will wait for the rays to hit something and come back. the time taken in doing so is calculated. And with this the distance of the object from the radar can be calculated. . This is used in army to detect enemy attacking and so on..
This is the simplest way..
ok thanks
The Brainliest Answer!
Radar means an instrument to see the flight of the aircraft on screen.
1 5 1
a system for detecting presence direction distance and speed of air craft ship and other objects by sending out pulses of radio waves which are reflected off the object back to the source
i wrote it correct