You're asking about stealth, but lets describe it easy.

Think about radar cross section like this: I put on a blind fold and I throw a ball forward. If it hits anything, it bounces back, so I know there is at least something there. If I throw it and it doesn't come back, I can assume there is nothing there. thats how radar works, but with radio waves, not a ball.

The radar cross section, is basically like if there was a moving target when I throw the ball. Except instead of one ball, I throw like a hundred. With a hundred balls, I can hit it easily if its big, but lets say its super tiny, like the size of a bee, its gonna be way hard to hit, so I may never even know its there.

Ok, but even those hundred balls, eventually I will know its there, say 1 in every 1000 balls hits it but its gonna be really hard to figure out where its going and whats happening when only 1/1000 balls hit it.