The document discusses low coupling and high cohesion in object-oriented design. It defines coupling as the degree to which elements of a system are connected to each other, and explains that low coupling is preferable to avoid issues with changes. It then defines different types of coupling between objects, such as data coupling and method calls. The document also defines cohesion as how related the functions of a component are, and that high cohesion is preferable to keep components focused and understandable. It recommends assigning responsibilities to achieve both low coupling and high cohesion.