First thing, that's a common misconception that the plane flies due to the Bernoulli effect.
See the these questions: What really allows airplanes to fly? Why does the air flow faster over the top of an airfoil? for the correct explanation.
Assuming that the bernoulli effect does explain flight, the answer to your question is:
Pressure of a fluid is force exerted by a fluid on a unit area of a neighboring body (the force exists inside the fluid as well, but it is balanced). So, if there is more pressure underneath, the upward force is greater (area if top and bottom of wings are approximately the same)
If the upward force is greater, the net force due to pressure is upwards. This force is called 'lift', and it balances gravity, helping the plane fly.
Read http://enwp.org/Pressure for more info.