Map 3D space to 2D screen space?

Question

Could some one help me to understand this code and explain it?

It's about converting from 3D space to 2D position, so that I can use it in another Effects (.fx) files, but I am a bit confused.

float viewportWidth;
float viewportHight;

//  calculate the 2D screen Position of a 3D position
float2 postProjectionToScreen(float4 position)
{
    float2 screenPos = position.xy/position.w;
    return 0.5f *(float2(screenPos.x,-screenPos.y)+1);
}

//  Calculate the size of one half of a pixel , to convert
//  between texels and pixels
float2 halfPixel()
{
    return 0.5f/float2(viewportWidth,viewportHight);
}