The Directional Derivative is a concept in calculus that measures how a function changes as you move in a specific direction from a given point. It provides a way to understand the rate of change of a function, such as f(x, y), in any direction defined by a vector. This is particularly useful in fields like physics and engineering, where understanding how quantities change in space is essential. To calculate the directional derivative, you take the gradient of the function, which contains all the partial derivatives, and then dot it with a unit vector that indicates the direction of interest. This process helps in optimizing functions and analyzing surfaces, making it a valuable tool in multivariable calculus and related disciplines.