Complex sentence: Demand-side learning refers to open source software's ability to be modified by the user community at large as needs and wants a those who use the software at no charge, they have the incentive to modify the software and learn about how it works.
Split into simpler sentences: Demand-side learning refers to open source software's ability to be modified by the user community at large as needs and wants arise. From those who use the software at no charge, they have the incentive to modify the software and learn about how it works.