On the screen you'll see a triangle. You can change this triangle by clicking and dragging the vertices (corner points) with the mouse. On the right a calculation of the area of the triangle is done for you.
You can choose which of the three sides you want to use as the base, by checking one of the three boxes at the bottom. The perpendicular height is then drawn relative to your chosen baseline.

The point of this exercise is to show you that the area of the triangle does not depend on which side is the base - you can choose any side and get the same result.
Play around until you are sure that you understand how base, height and area relate to one another.