Normal force

Normal force is the force that the ground (or any surface) pushes back up with. If there was no normal force, you'd be slowly seeping into the ground.

On a flat surface, the normal force of an object is $mg$.

On an inclined plane, the normal force is reduced by the angle, and the normal force is $mgcos\theta$.

Note that on a flat surface, $\theta$ would be 0, and so $cos\theta$ would be 1. So the two equations are equal.