Ecosystem services are the numerous benefits that humans derive from nature. These services can include both market commodities (e.g., timber and fish) and non-market services (e.g., flood control and pollination). In the last three decades, an ever-increasing body of research has sought to define, understand, and put a price tag on these services in order to convince lawmakers and the general public that ecosystems need to be better protected. This video, prepared by the Natural Capital Project, provides an excellent introduction to ecosystem services and why researchers are working to estimate their values.
Wikipedia also offers an accessible yet detailed introduction to this concept.