A density curve is used in statistics to make a rough illustration of a distribution. It is always drawn above the horizontal axis on a graph, and the area is always one.
The area below the density curve represents all observations that fall within the range of the horizontal axis. It is an idealized picture of the data, and is not perfectly accurate. If the density curve is symmetrical, then the mean and the median will be equal. If the distribution skews left, then the mean will be less than the median, and vice versa if the distribution skews right.