okay basically there is pressure everywhere. pressure is pushing down on the water in the glass, pressure is pushing inside and outside the straw, etc. When you suck on the straw, you are reducing the air pressure inside of the straw, and to balance out this pressure from the pressure pushing on the liquid in the glass, water comes up in the straw. If there is a hole in the straw, the water wil not come up because the pressure balances itself out with the hole.