Nonprofit ABC. reports that they served 100 children in year 1; 1,000 children in year 2; 100,000 children in year 3. So what is the result of your service? How are the children better off because of the nonprofit ABC work.
Outputs measure how much you did and this is important but not nearly as much as the results of what you did. The results or the outcomes of what you did answer the question So what.
What value does your nonprofit produce and how do you prove to stakeholders and funders?
I suspect many nonprofit make outcome or program evaluation much more complicated than it has to be. You are not writing a book or a Phd dissertation. You are often merely asking people if they are better off because of your services. You can measure this in many ways such as by asking the people who serve directly or maybe ask them to complete surveys to determine if they increased their knowledge or if they are changing their behavior because of your service.