Why it matters.
It helps explain why a brightened low-light picture can look rough.
A closer look.
Noise comes from variation in captured light and electronic processes. Gathering more light can improve the signal relative to noise; noise reduction trades against fine detail.
Picture this
Compare a well-lit shadow with a very dark shadow brightened in editing. Look at texture as well as grain.
Keep in mind.
A noise-free appearance does not prove that detail survived. Strong smoothing can remove both noise and useful texture.