HDCP stands for High-bandwidth Digital Content Protection and exists to prevent copying of digital video and audio content across HDMI, DP, DVI and other connections.
HDCP prevents encrypted content from being played on unauthorised devices. The content source queries the receiver to make sure it is authorised before allowing the content to be shown. Older DVP devices may play from a DVI source, but cannot play content using a modern HDMI source because of HDCP.
HDCP can cause technical issues when AV content is being shown and split between screens using HDMI/DVP cables. Scalers, switchers and splitters are also effected by this.