A Riemann Sum is a method used in calculus to approximate the area under a curve. It involves dividing the area into smaller rectangles, calculating the area of each rectangle, and then summing these areas. The height of each rectangle is determined by the value of the function at specific points, such as the left endpoint, right endpoint, or midpoint of each subinterval. As the number of rectangles increases and their width decreases, the Riemann Sum approaches the exact area under the curve, which is represented by the definite integral. This technique is fundamental in understanding how integration works and is widely used in various applications, including physics and engineering.