It works exactly like you looking around a room. Large objects take up more space in your vision so it's easier to see. Smaller objects are harder to spot. Transparent objects are even harder to see.

Radar works like your eyes, except you can't really see the details of the object. Just how big it is, like you are really near sided and everything is just blurry shapes in the distance. With stealth, they shape the object to make it mostly transparent to radar, like a glass window for visible light. They also use radar absorbent materials, which is like painting an object black to hide it in a dark room. Both of these reduce the radar cross section.

Radar cross section could be used to approximate what a jet would look like on radar relevant to other things and basically describes how easy it is to spot on radar.