The document discusses the concept of urban green infrastructure. It defines urban green infrastructure as the parts of a city that contribute to natural processes like keeping water and air clean and recycling waste. This includes parks, stream corridors, utility corridors, and vacant lands. These green spaces, if viewed as a single interconnected system, can help keep cities clean and provide recreational areas. The green infrastructure works with engineered systems to manage resources like energy, waste, and stormwater runoff in a balanced way, analogous to natural ecological processes.