Energy density refers to the amount of energy stored in a given volume or mass of a substance. It is an important concept in fields like physics and engineering, as it helps us understand how much energy can be extracted from fuels or batteries. For example, a high energy density means that a small amount of fuel, like gasoline, can produce a large amount of energy, making it efficient for powering vehicles. In contrast, materials with low energy density, such as batteries, may require larger sizes to store the same amount of energy. This is crucial when designing devices like electric cars, where space and weight are limited. Understanding energy density helps us choose the right energy sources for various applications.