The Super Game Boy is an adapter cartridge for Nintendo's Super Nintendo Entertainment System, as well as the Super Famicom in Japan. The Super Game Boy allows game cartridges designed for use on the Game Boy to be played on a TV display using the SNES/Super Famicom controllers. When it was released in 1994, the Super Game Boy sold for about $60 in the United States. In the United Kingdom, it retailed for £49.99 It was the precursor to the Game Boy Player on the Nintendo GameCube, which functioned in a similar manner.
It could only play Game Boy Color games that came in Black Cartridges. As these unique cartridges had a "Dual-Mode" which allowed them to play in Classic Monochrome Gameboys. It is this mode that loads in the Super Game Boy.
Source: Wikipedia, "Super Game Boy", available under the CC-BY-SA License.