A path tracer makes a picture by following light the way physics does: fire a ray, let it bounce off surfaces, and add up the light it carries. One ray is a wild guess — the image is the average of thousands.
Because it is an average of random samples, the grain falls only with the square root of the count. Halving the noise costs four times the paths; that steep exchange rate is why rendered films still take hours a frame.
Every bounce multiplies the light by the surface it left. A red wall multiplies the green and blue away, so light leaving it is tinted red — that stain on the nearby floor is colour bleeding, and it comes for free.
Most of what you see is indirect light — it bounced to reach your eye. Hide the lamp behind a wall and the room still glows, softer and tinted. Cap the bounces at one and that whole world of soft, filled shadow simply switches off.
Something in the simulation stopped unexpectedly — the lesson continues without it. Nothing you did was wrong; you can move on.