What is bitrate, and why does it matter more than resolution?
Bitrate is how much data is used per second of video — and it frequently determines picture quality more than resolution does, which is why a 4K stream can look worse than a Blu-ray at 1080p.
The relationship. Resolution sets how many pixels there are; bitrate sets how much information is available to describe them. A high resolution with insufficient bitrate means each pixel is described poorly, producing compression artefacts — blocking, banding in gradients, smearing in fast motion, and loss of fine texture.
Where you see it most: dark scenes, where subtle gradations become visible bands; smoke, water, rain and foliage, which contain vast amounts of unpredictable detail and are expensive to compress; and fast camera movement.
Why streaming looks worse than physical media. A 4K Blu-ray may use a bitrate many times higher than a 4K stream of the same title, because the disc is not constrained by anyone's broadband connection. The resolution is identical; the information is not.
Constant versus variable bitrate. CBR maintains a fixed rate throughout, which wastes data on simple scenes and starves complex ones. VBR allocates more to demanding scenes and less to static ones, giving better quality for the same file size — and is standard for distribution.
Adaptive bitrate streaming is why quality changes mid-programme: the player continuously measures available bandwidth and switches between versions encoded at different rates, which is preferable to buffering but produces the visible drop in sharpness.
Codec efficiency changes the arithmetic. A newer codec achieves comparable quality at a substantially lower bitrate, so bitrates cannot be compared across codecs.
Audio bitrate works the same way, and modern codecs achieve transparency at far lower rates than older ones — which is why the familiar figures associated with MP3 are not a useful guide to what a modern codec needs.
The practical implication: when choosing settings, raising resolution while leaving bitrate unchanged usually makes the result worse, because the same data is spread across more pixels.