Two copies make a parallelogram of base (a + b) and height h. So one trapezium = half of that.
Drag the brass dots to change the two parallel sides, and the mustard dot to change the height. Hit duplicate & rotate: a second copy spins around to sit on top, and together they form a parallelogram of base (a + b). One trapezium is exactly half of it — that's why Area = ½(a + b) × h.