What does "net effect" mean? (Note the word order.)
It's the final effect resulting from a combination of many different
effects, some of which add to the final effect, some of which detract
from it.
It's the final result after all the positive and negative components are combined.
A similar expression is "net pay". On your paycheck you have a
gross amount, based on your stated salary, and a
net amount, which is the gross amount minus taxes and other deductions. The net amount is also called net pay.
CJ