An area chart is a line chart where the area between the line and the axis are shaded with a color. These charts are typically used to represent accumulated totals over time and are the conventional way to display stacked lines.
In Line chart the marks(connecting points) are calculated from the base line i.e X axis all the time.
Suppose month on month you have birth rate for male and female. And on gender basis you are creating a line chart .
Month MaleBirth FemaleBirth 1-Jan-1946 27 18 1-Feb-1946 24 18
In case of line chart the point for male of 1946 Jan will be calculated as 27 from X axis and Female 18 from X axis.
But in Area chart the point(mark) is not calculated from the base line (X axis) all the time.It will get calculated from the point of below chart. This is called stack which means one over the other.
Suppose the male birth area is calculated first. In that case each point will be calculated from base line for male. SO area chart for male Birth will have the point from X axis at 27. But for Female It will be calculated from the male birth line. So it will be at point 18 above the male chart for month Jan 1946 i.e 27+18 = 45 from X axis.
Hope this answers your question.