mgcos(theta) is the normal force on an object atop an inclined plane.
mgsin(theta) is the component force of gravity on the object parallel to the plane. In the sense that there is no upward (up the plane) force on an object, and that the component force of gravity perpendicular to the inclined plane (equal to the normal force, mgcos(theta)), the mgsin component of gravity can be considered the net force on the object.