Basic Physics
AF330 - If you want to better understand how airplanes fly and maneuver I suggest that you find a very basic book on the subject. There are many available on line.
For the very basics of trim and balance here are a few thoughts:
In order for any object to be in steady state equalibrium the forces and rotational moments applied to that object must balance. The sum of the forces must be zero as must the sum of the rotation moments be zero.
As for up/down forces on the tail, these generate rotational moments. A tail down force causes the airplane to rotate in the tail-down / nose-up direction. This is what BuzzBox is referring to. If the airplane has a tendency (due to other rotational moments) to rotate in the nose down direction, down load on the tail is needed to bring it into rotational equilibrium.