The document discusses the midpoint theorem, which states that the line segment joining the midpoints of two sides of a triangle is parallel to the third side and half its length. It provides examples of using the midpoint theorem, such as proving that the line segments connecting the midpoints of opposite sides of a parallelogram trisect the parallelogram's diagonal. The midpoint of a line segment is the average of the coordinates of the segment's endpoints. The midpoint theorem has applications in fields like construction that involve measuring heights and areas.