Actually, what they mean, is that generated + imported power within thier controlling region has a cushion - There is more available than is being used. When the power plants reach 100% and the incoming paid power is used 100%, then there's 0 reserves. If they continue to operate on that, an increased demand will do one of two things: trip breakers, causing massive and destructive overloads and outages, or voltage drops, which again, cause problems.
You can't overload the generation facility or cause main trunk lines to be overloaded, becuase the entire nation's power system is interconnected. They must maintain fully accurate 60 cycle sync with everyone else, and they must maintain the voltages, or suddenly, they'll have destructive transients across the grids. That happened a few years ago, during a huge storm, and it took the power down in about 15 if memory serves. It took several hours to restart the nation's power grids once that happened. I seem to recall that all of CA, Arizona, nevada, utah, OR, WA, ID, and some of Montana and colorado went dark almost precisely at the same time. Later, it was determined that somehow, due to some regulation error, parts of one power grid got out of 60 cycle sync with the rest, causing the interties to overload and then knocking down most of the west. They then had to bring it back up a single section at a time, syncing all the power generation stations one at a time to the (international, actually) grid.
So, when they say that their reserves are low, it means that thier ability to regulate the conditions of thier power delivery have reached an almost-failure point. If they failed, it is possible, perhaps even likely, that not only CA would have problems, but it could take down the power transmission temporarily in a large section of the country.
So this time the enviro wackos have not hurt just the people they live near, but it has the potential for doing damage to the rest of US as well.
Attention: TDR Forum Junkies 
