Density describes how compact or concentrated something is. For example, suppose you have two boxes, one large and one small. However, they both weigh the same. That means the small box has a higher density than the large box.
Density also tells how concentrated or crowded something is. You may have heard of population density. In a city, there are many people packed into a small area, giving it a high population density. In a rural area, there are more fields or wooded areas between houses. That means there are fewer people living in a larger area, which is why it has a low population density.