matrix transformation matrix transformation

[adnansojib]

build matrix then invert move to origin then compare and then back original position__ i dont understand this thing _please help me 

float sectdists[] = point(1, "sectdist", @id);
matrix xforms[] = point(1, "xform", @id);

foreach(int i; matrix xform; xforms){
matrix xform_inv = invert(xform); /// please 
vector local_P = @P*xform_inv; ////please 

if (local_P.z > sectdists){
local_P = set(local_P.x, local_P.y, sectdists);//// please 
@P = local_P * xform;

}

}