If you push a heavy box which is at rest, you must exert some force to start its motion. However, once the box is sliding, you can apply a lesser force to maintain that motion. Why?

When we try to push a heavy box which is at rest and as we know that when an object is at rest the frictional force acting on that is static friction which is not constant and static friction is equal to the applied force and increases as we increase the applied force till the object just starts to move and the maximum value of the static friction is greater than the value of sliding friction which is constant and occurs when the object starts sliding or moving properly. The magnitude of sliding friction is always less than static friction.