Some streaming videos are protected by HDCP (High-bandwidth Digital Content Protection). This means that you can only watch them using secure connections that are designed to prevent video copying. If you try to watch one of these videos using a connection or device that isn't HDCP-compliant, you'll get the following error:
HDCP restrictions are preventing this video from playing. If you are attempting to watch on a secondary external screen or device, try disconnecting it and watching the video on your main screen.
This may happen if you're trying to watch an HDCP-protected video using a laptop that connects to an external monitor, for example.