Population density refers to the number of people living in a specific area, usually measured per square kilometer or square mile. It helps us understand how crowded a place is. For example, a city like New York has a high population density because many people live close together, while a rural area might have a low population density with fewer people spread out over a larger space. High population density can lead to challenges such as traffic congestion and limited resources, while low population density might mean more open space but fewer services. Understanding population density is important for urban planning and resource management in places like Tokyo or Los Angeles.