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 fall 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 acceleration due to gravity).

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

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