The HKT-8100 allows your Dreamcast to be connected to either a VGA screen, using a D-Sub cable, or to a TV using either the classic red-white-yellow cable or the S-Video 4-pins connector.
Some Dreamcast games get a serious boost in graphics, using the "VGA Box".
Others, such as Street Fighter 3, appear graphically downgraded, because they're optimized for 2d rendering.