The Hydrogen atom consists of a single Proton at its nucleus, and an Electron orbiting it. The closer the electron is to the nucleus, lesser the energy it has. The lowest energy state (according to Quantum Mechanics) that the electron can have, is called its ground state, when the electron is supposed to be closest to the nucleus.
However, according to Randy Mills and co-workers at BlackLight Power, a company based in Cranbury, New Jersey, there might be a still lower energy-state, which they have termed the hydrino state:-?.

Left: Shrunken Hydrino State, Right: Normal Ground State (Courtesy: Blacklight Power)Mills argues that the hydrino state could be used as a new source of energy - a claim that has led to a predictably negative response from other researchers:). Termed it the Blacklight process, the claim is that the process allows the electron to move closer to the proton, to which it is attracted, below the prior-known ground state. According to them, this generates power as heat, light, and plasma (a hot, glowing, ionized gas) with the formation of strong hydrogen products that are the basis of a vast class of new chemical compounds with broad commercial applications.
Earlier this year, Andreas Rathke of the European Space Agency published a paper in which he argued that the theory for the hydrino state put forward by Mills was the result of a mathematical mistake. Now another theorist has joined the debate: Jan Naudts of the University of Antwerp in Belgium argues that the Klein-Gordon equation of relativistic quantum mechanics does indeed permit the existence of a low-energy hydrino state!
A video explaining the Blacklight process is described here. Only time will tell. I, for one, am quite skeptical. Another skeptical look is provided here.