It's a calculated report amount that includes all prior years' net of revenues and expenses.  

It's an opening balance, so if the current year is 2014, it is the opening Retained Earnings balance for 2014, the ending balance sheet / closing balance for 2013. 

If you do a comparative trial balance of the prior two years (i.e. if the current year is 2014, for 2014 and 2013), with 2013 on the right, the amount of income from 2013 be the difference between the two starting amounts.