This is about the fact that the so-called "renewable" energy sources of biomass, wind, solar direct or PV, and even hydro (snow and rain), are not sustainable or reliable enough for a world population of 7,000 million.
For instance, the maximum biomass that can be grown sustainably (i.e. without dependence upon fossil fuel for fertilizer) to supply energy at the rate of just one personal automobile getting 30 miles per gallon of gasoline, and driving 15,000 miles in a year, requires about 1.3 acres, not counting the energy needed for maintenance and harvesting. My fairly generous suburban plot is about a quarter of an acre, or less.
As for wind, the entire wind turbine capacity of E.ON in Germany, 12,000 MW, according to their records, dropped in December 2008 from a peak of 12,000 to less than 3,000 for three whole days. The rate at which it varied, a serious problem for scheduling replacement power, is shown in the graph.
But the real alternative to the fossil carbon that displaced the 18th century technologies is the energy of the atomic nucleus. At present, reactor designs that are over 40 years old provide 20% of the USA's electrical energy demand, with a total of under 3000 tons of spent fuel per year, which is erroneously labeled "waste". An equivalent generation of energy from coal or even "natural" gas releases to the atmosphere tens of thousands of tons of acidic, poisonous gases, not counting the millions of tons of CO2.
There are already two versions of clean, safe, renewable, and sustainable nuclear fission technology. They have both been well demonstrated experimentally for over 30 years each. Note that "experimental" scale in a reactor has the capacity equivalent of half a dozen maximum size wind turbines.
My main website is at http://energy.skepticva.org
