Question

explain why atmospheric pressure is less at the top of a mountain in comparison with sea level

Answer 1

because as we go higher and higher the altitude increases as well as the atomspheric pressure decreases that is we get more oxygen here and on the hills less oxygen

Answer 2

Air has a weight too, although not very much, If you "pile" the air mile high, the bottom pressure is heavier because of all the air sitting on top of it, therefore the pressure decreases with altitude, because there is less air "piled up" 
An analogy would be the same with water.