Mills's theories originally developed out of his interest in cold fusion, though he insists he's not proposing a rejiggered form of cold fusion. Instead, what Mills claims to have discovered is a way to get a hydrogen atom to move to an energy level below the ground state. The ground state is the lowest energy level a hydrogen atom can sink to (according to modern physics). But Mills is saying it can sink even lower (i.e. the electron can move even closer to the proton). When a hydrogen atom sinks to this sub-ground level, it supposedly emits an enormous amount of energy and transforms into what Mills calls a "hydrino". If Mills is right, pretty much all of modern physics is wrong. Which is why Mills probably isn't right.
Of course, Mills could be a genius whose theories are going to completely revolutionize modern science (and modern industry). That's what his supporters claim. But that's what the supporters of ALL free-energy schemers claim. The fact is that for almost fifteen years Mills has been promising that practical applications of his hydrino technology are just around the corner. But nothing ever materializes. And meanwhile he keeps luring in new investors with his wild promises of limitless energy. So it seems to me that Mills and his hydrinos match the familiar free-energy pattern of big promises, but no results.