It uses the same method as in
Area of a polygon but does the arithmetic for you.

Instructions

Enter the vertices in order, either clockwise or counter-clockwise starting at any vertex.

Enter the x,y coordinates of each vertex into the table. Empty rows will be ignored.

Click on "Calculate".

Unlike the
manual method, you do not need to enter the first vertex again at the end,
and you can go in either direction around the polygon.
The internal programming of the calculator takes care of it all for you.

There are other, often easier ways to calculate the area of triangles and regular polygons. See