Population density refers to the number of people living in a specific area, usually measured in people per square kilometer or square mile. It helps to understand how crowded or spacious a place is. For example, urban areas like New York City typically have high population densities, while rural areas may have much lower densities. High population density can lead to various challenges, such as increased demand for housing, transportation, and public services. Conversely, low population density might result in less strain on resources but can also lead to issues like limited access to amenities and services in places like Montana.