Radar works by transmitting and observing the reflections of radio waves. Which are electromagnetic waves not unlike visible light, except in a different frequency, so we can use a visible light metaphor.

Imagine you are in the middle of a vast open field, and it's the middle of the night: there is no ambient light. But you have been equipped with a powerful flashlight, which you can point in any direction you like. You are also equipped with a partner we'll call Sam. Sam will go where you point if you command him to, and he's a very fast runner, but it takes him a few second

There are five enemies out there. Their job is to tag you out, but they lose if Sam tags them first.

Assassin 1 is vastly obese and is wearing a cheap white shirt. You spot him easily and Sam tags him out: he has an immense cross section.

Assassin 2 is just as obese but is wearing all black. Your flashlight does not reflect off him as well, but as he is such a large and slow moving target you have no problem seeing him in time to send Sam after him. His physical cross section is the same but his visual cross section is far less.

Assassin 3 is very athletic- not as fast as Sam, but damn fast. Still didn't pay attention to his wardrobe, though: his white shirt gives him more than enough cross section for you to see him and Sam to run him down before he reaches you.

Assassin 4 is like 3 but wearing all black. You barely spot him in time.

Assassin 5 came prepared. He spent a lot of time, money, and effort to camouflage himself. He can't change his physical cross section, but visually, you can't detect him because none of the light from your flashlight is returning to the detector-your eyes- in a way you recognize as a meaningful object.