The area of a rectangle is width × height. If you have a rectangle with width 5 and height 3, the area is 15.
When a rectangle is given by coordinates, the width is the horizontal distance and the height is the vertical distance. For corners at (x₁, y₁) and (x₂, y₂), width = |x₂ - x₁| and height = |y₂ - y₁|.
Area = |x₂ - x₁| × |y₂ - y₁|. The absolute values handle rectangles in any quadrant.