If the plane is parallel to the ground, the force of gravity acting on the box would be perpindicular to the plane. No force would be required to keep the box from moving in a direction parallel to the plane.
If the plane is at an angle to the ground, how much of the force of gravity acting on the box would be in a direction parallel to the plane? That is the amount of force that you would have to apply to the box to keep it from sliding down the plane.