Answer:
I would assume that the box slows down due to the frictional force between the floor and the box. In addition, the frictional force points opposite of the boxes motion. As we know, the box was given an initial speed, therefore, having an applied force. Hence, we can conclude that the frictional force opposing the motion of the crate is equal to the applied force but acting in the opposite direction. This frictional force is called static friction. When we increase the applied force (push harder), the frictional force will also increase until it reaches a maximum value.
Hope that helps!