Density nothing other than a way to describe how much of something there is inside something else. In most instances of life, when someone asks for the density of something, we want to know how much weight is inside a certain amount of space. How much mass is in a given volume? This is usually measured in grams per milliliter, g/mL (or grams per cubic centimeter, g/cc, which is the same.)

If you have two bottles of the same size, one filled with water, and one is empty (filled with air), the bottle filled with water is heavier when you pick it up. This is because water has a higher density than air, so even though it takes up the same volume, it is much heavier than the air in the other bottle. (This is also why water always falls towards the floor, because if it was less dense than air, water would float away, and the air would "sink".)

However, I mentioned you can talk about _anything_ inside a given volume. This leads to many other, less common types of density, such as power density or energy density (often used for batteries to describe how quickly you can discharge them or how much you can get in a given volume or size), number density (how many things in a given volume), or population density (how many people live inside a given area).