OK, Here's the explanation in a nut shell.

Gravity is a constant. It exerts pull on all objects in one direction. Straight towards the center of the earth.

Now. if you have a wind of constant speed at full value (let's say 90 degrees from the left going to the right) that wind is going to drive your bullet to the right. If you fire a bullet directly sideways of the wind, it has the greatest effect on that bullet.

If you shoot at a 45 degree angle to the wind we call it a "half value wind" because the wind is only exerting 1/2 of it's force to the side of the bullet. If we shoot at an 22.5 degree angle to the wind, it's a 1/4 value wind, and if we shoot directly away from the wind it's a zero value wind.

Gravity works the same way, for basically the same reason. If you are shooting down hill at 45 degrees, gravity is only effecting the bullet at that same 45 degree angle. Not at 90 degrees as it would be on the firing range.
Now, this is a bit oversimplified, because of other factors that come into play, (altitude, temp, humidity, ballistic characteristics of the bullet itself and so on) but this is the basic concept.