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 [math]\displaystyle{ mg }[/math] (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 [math]\displaystyle{ mgcos\theta }[/math].
Note that on a flat surface, [math]\displaystyle{ \theta }[/math] (angle)would be 0, and so [math]\displaystyle{ cos\theta }[/math] would be 1. Thus, the two equations are equal.