HDCP (High-bandwidth Digital Content Protection) encrypts the video signal on the physical link - HDMI or DisplayPort - between a playback device and the screen, so the decrypted picture cannot be captured by a device spliced into the cable. It is the last leg of content protection, beyond DRM's job of protecting the stream.
DRM license policy frequently mandates a minimum HDCP version (e.g. HDCP 2.2 for 4K/UHD). If the connected display or cable does not satisfy it, the platform must downgrade resolution or refuse playback. This is why a 4K stream may drop to HD on an older TV or through a non-compliant adapter - the entitlement and license layers are enforcing the studio's output-protection requirement.
