What exactly is the engine performance index?

I’m doing the scenarios and they have certain “engine performance index” points you need to have. i.e. engine performance index higher than 107. What does that mean? Horsepower?

General power. A lower power car with a wide power band will have a higher performance index than a higher power car that’s peaky.

It is the average power (in kW) that is produced in the top-something-percent of the rev range. As such it is exactly as @Kyuu77 is saying, and it is a pretty good indicator of how much effective power the car has.