Radar is used by militaries and civilians of the world for object detection.
Radar works when a tower shoots a "beam" of radio waves in a direction. If an object is in this "beam" of radio waves, the waves will bounce back to the tower.
The owner of the radar tower receives two very important types of data from the use of radar: Distance and velocity.
Distance between the radar tower and object is determined by the time it takes the radio waves to return to the tower after they are initially shot. The radio waves travel at light speed. Therefore, it's pretty easy to determine the distance. Take light speed, multiply by the time for the round trip, and you get the distance. there is one twist, however. The total distance must be divided by two because the radio wave made a round trip, going to the object and back.
The velocity of the object in the radar beam can be found using the Doppler effect. If the object is moving away from the tower, the frequency of the returning radio waves would drop. The opposite is also true. If the object is moving towards the tower, the frequency of the returning radio waves would go up. The extent to which the radio waves are shifted helps pinpoint the objects velocity.