Normal force

F_N represents the normal force

Normal force is the force that the ground (or any surface) pushes back up with; If there was no normal force, you would slowly seep into the ground.

The normal force on a object is always perpendicular (at a right angle) to the surface the object is on.

On a flat surface, the normal force of an object is $mg$ (the object's weight, that is its mass multiplied by the force of gravity).

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, $\theta$ (angle)would be 0, and so $cos\theta$ would be 1. Thus, the two equations are equal.