If you look at the results the ROI shows the ROI of the campaign. We calculate it the following way according to the industry standards:
Revenue/Add spend * 100 = ROI
It is also called ROAS and shown in %. The number states how much money you got back in comparison to what you invested. For example if the ROI is in
0 = nothing
100% = I got everything back I invested
200% = I got back twice as much as I invested