A misconception. It is supposed to be a combination of heavy metal and rap music, but these genres are fundamentally different. Rap focuses primarily on rhyming vocals with a simple and repetative beat in the background. Metal is the combination of fast and complicated instrument work with intense vocals. Hence the two genres cannot mix.
People say crap bands like Linkin Park, Papa Roach and Rage Against the Machine are rap metal. None of these bands have an ounce of metal in them, and probably fail at rapping too. They are prime examples of undertalented, overwhore MTV bullshit.