A PRO System's ROI is referring to Return on Investment, or in other terms it’s profitability over time. This metric is calculated by taking the money won from the specific system, dividing it by the money risked and multiplying by 100. 
The Bowl Season Betting Against PRO System shown above has an ROI of 15%, which is very strong given the large sample size of games played. This comes from the 19 units won divided by 123 games played (pushes are excluded), which gives us .1545. After we multiply .1545 by 100, so we get 15.45, which can be rounded to 15%.
There is no specific number to look for in terms of ROI, but most PRO Systems will feature a ROI above 5%.