The world needs a new source of energy, an unspillable source.

who could explain what is zero reference point in conservation of energy?

i have some problems regarding to the conservation of energy. assume we take somewhere as zero reference point, does it mean potential energy(mgh), h=0? and also, h means the height from where to where?
i need a clear explanation, it will be much better with examples relating to this problem. my exam in one week, please help thanks! : )