The .BIN extension is used very widely for many completely unrelated data types. It happens to be a very common extension for ROM images of many formats. In the context of Genesis ROM images, a .BIN file is a straight binary image of the cartridge ROM contents rather than, for example, a SMD image, which has an additional header and organizes the data in interleaved blocks (probably to optimize for the communication protocol on SMD copiers). You can load it directly into just about any Genesis emulator to play it, or if you need it to be in a different format e.g. for use on a copier there are tools that can convert between formats.