RSA is a fancy word for the type chq sum that is needed in the file to make the file "compleate", if the chq sum is not compleate or correct the car wont run.
RSA is a more complex type of chq sum compared to some other ecu files,
and if it fails or is incorrect it can do wonderfull things to the rest of the ecu memory and components in the car that communicate to the engine ecu.
Not a very complex explantion from me but it gives you an idea