Density curve

A density curve is the idealized version of a histogram: you stop reading heights as counts and start reading area as proportion. The total area under the curve is exactly $1$, and the area above any interval is the proportion of the data, or the probability, falling in that interval. For example, a uniform density curve spread over 0 to 4 has constant height $1 / 4 = 0.25$ because $4 \times 0.25 = 1$, so the area from 1 to 3 is $2 \times 0.25 = 0.50$ and exactly half the values lie there. Every normal distribution problem you solve is an area-under-a-density-curve problem.