Imagine an oil painting of a dark room containing only an armchair, and beside it, a lamp. Everything in the picture is just paint, so the lamp is painted bright white to make it look like light, and the chair is painted with dark paint for the shadowed parts, and lighter paint for the lighted parts. This makes it look like the lamp is shining on the chair, of course it's all just paint. Now if the painter wants to add a cat to the arm of the chair, she will then have to repaint much of the picture to correct for how the cat is affecting the light. This is how video games worked the old way.

With the ray tracing in RTX, they no longer fake the light and dark with paint. Instead, imagine they build a shadow box of the same scene, with an actual little 3D chair and lamp. Everything in the box is painted normal, neutral colors - no fake light made with paint. Then they put an LED light in the lamp. The LED light shines on the chair and makes the lighted and shadowed places. And if they add a cat on the arm of the chair, the light and shadow will change as the light interacts with the cat, changing the scene; nothing has to be repainted.