The under a density curve above a range of values tells us the proportion of the population with those values.

area

Explanation:

A Density curve is a graph which shows probability of something. The area under a density curve above a range of values tells us the proportion of the population with those values. In other words this represents the 100% of all the probabilities that exist, which is why it equates to 1, since in decimal form 1 would be 100%. An example of a density curve can be seen below.