A PRO System's 'ROI' is its return on investment. This number is calculated by taking the money won, dividing it by the money risked and multiplying by 100.
The Windy Under system above has an ROI of 11%, which is very strong given the large sample size of games played. This comes from the 89.99 units won divided by 851 games played (pushes are excluded), which gives us .1057. After we multiply by 100, we get 10.57, which is rounded up to get 11%.
There is no "magic number" to look for in terms of ROI, but most PRO Systems will feature ROIs above 5%.