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.