Organic growth is the increase in profits generated by the operations of existing stores, the stores you owned on for the entire year.
In 2016, John’s drug store made a profit of $75,000. In 2017 it made a profit of $80,000. In April of 2017 John opened a second store that generated a profit of $160,000. John’s total profit increased from $75,000 in 2016 to $240,000 in 2017 ($80,000 + 240000), a total increase of $165,000 ($240,000 -$75,000). Of that $165,000 increase only $5000 was organic growth. Only $5000 of the increase came from a store he had owned in 2016.