Japan's electric infrastructure comprises two main power grids. One system, in the west of the country, operates at 60 hertz, like power in the U.S. The eastern parts of the country, where Tokyo and Fukushima are located, run on a 50-hertz system, like the one used in Germany.
Ordinarily, this isn't a problem — there are enough power plants in each of the grids that electricity can be shifted around if there are spikes in power demand or outages at a plant. There are also ways to pass some power across the 50-hertz/60-hertz divide between the power systems, but this is only available for a limited amount of electricity.
The trouble comes when there's a big, unplanned shortage of power, like what's going on now with the destruction of the Fukushima Dai-ichi nuclear power plant. Creating new linkages between the 50-hertz and 60-hertz systems is incredibly expensive and couldn't happen for years. So Tokyo Electric Power Co., or TEPCO, which operates the stricken plant, is rapidly trying to secure more power to make up for the loss in production. Until that happens, though, Tokyo could see more rolling blackouts or other measures introduced to reduce electrical demand.