...is a made up number that is meant to "inspire," according to its promulgators. As the Wall Street Journal reports:

The Apollo Alliance, a San Francisco coalition of environmental and labor groups, ... released a study in September. It concluded that five million green jobs could be had with an investment of $500 billion -- more than three times Mr. Obama's number [ of $150 billion].

Kate Gordon, co-director of the Apollo Alliance, says the numbers are less important than the message. "Honestly," she says, "it's just to inspire people."

Earlier this week, my colleague Jacob Sullum pointed out in his superb column "Green Herring": 

...both the Apollo Alliance and Obama, who has liberally borrowed from its ideas, mistakenly treat the manpower required to reduce greenhouse gas emissions as a measure of success, when it should be viewed as a cost to be minimized.

Whole WSJ article here. Sullum's column can be found here