ROI is basically a term that help us understand.
How much money we earned from an investment.
For example:
We invest $100 in a cfd position
We won a contract and earned $80
Overall, we got $180 back while investing $100
The formula to calculate ROI is
So in our case ROI is (180-100)/100 = 80%
There is more to understand ROI, but this is all you need to know for the basics.