Engineers must design structures so that they can endure different types of forces. One of these forces is called shear. Shear is a force caused by an external load which causes two segments of a body to slide over each other. As demonstrated in the Shear video below, if you had a foam brick and pushed the top part to the left and the bottom part to the right, the foam brick would experience a shear force.
In order to be an effective engineer, one must be able to design against destabilizing forces such as shear.