When does a red screen occur?

I recently got a red screen while watching Netflix, which I had never had before. There was also no error message, just a completely red screen and a slight hum. What can be the causes of a red screen? And what does that mean? Is that an error message at all?

It's best to close the application or restart the device. Netflix has its moments

This can affect all streaming services that use HDCP copy protection. Often it is enough to start the application (in this case Netflix) or restart the device.