It's a government statistic. You take all the money earned from every source and you divide that number by the population. For the United States the GDP was about 19 trillion dollars and the population was roughly 330 million.
19 000 000 000 000
===============
330 000 000
~ about 60 000 dollars (this is just an approximation. I've rounded quite a bit.
So the per capita income is 60000 for every man woman and child in America.