Sealing the top of a straw with your finger stops air entering and exerting a downward force on the liquid, leaving only the upwards force of air pressure from below.
This upwards force is stronger than the force of gravity pulling down on the liquid. Take your finger away from the straw and the downward force of the air pressure on the liquid will be the same as the upward force.
These two effects cancel each other out leaving gravity as the dominant force causing the suspended liquid to drop out the bottom.
Answered by Rik Sargent