HDCP is "High-bandwidth Digital Copy Protection" and it is used to provide copy protection security in digital connections such as DVI and HDMI. HDCP-compliant devices are classified as sources, displays, or repeaters (a classification that includes switches). If the source is HDCP compliant, it will not output video until it has established an HDCP "handshake" with the the devices downstream of it to make sure that you aren't somehow stealing that raw digital data in order to pirate the video.

Unfortunately, it has proven consistently difficult to implement effectively - issues like the sequence in which devices are turned on can make or break the handshake.