If you see a reference to a D-Sub signal input on a projector or LCD television, it refers to an incoming VGA, or video graphics array, connection. These would typically be used to connect a computer or laptop.
D-Sub refers to a "digital subminiature" cable connection. The VGA D-Sub cable uses a D-shaped connector containing 15 pins.
You should insert your VGA D-Sub connector into the appropriate port on your TV or projector and fasten it with the thumbscrews on the connector. Ensure the display resolution on your input device is configured to a setting that is compatible with your screen.
Other connectors can be used to connect computers to output devices. These commonly include the DVI, or digital visual interface, connector and the HDMI, or high definition multimedia interface, connector.