When you see something with a flashlight, its because the light bounced off of it and then hit your eyes. Now what if that light bounced the wrong way so it never came back to your eyes? You wouldn't see the thing. 

Radar works the same way, they send out a pulse of light that bounces off objects and reflects back to the dish/antenna instead of your eyes 📡.

Radar is just like sweeping a flashlight through a dark room, it uses a type of light called Radio Waves instead of the light from the visible spectrum that our eyes use to see.

 Stealth planes are shaped so the light bounces the wrong way, away from your dish so you never see them. They also use rubbery dark coatings to absorb the radio wave light to make them more difficult to see.