Your airspeed indicator, altimeter and vertical speed indicator all work with air pressure. That pressure comes in through two openings: the pitot tube and the static port.

The pitot tube points forward into the airflow, often under the wing. In it you measure the total pressure: the pressure of the air around you plus the pressure of the air you fly into. The static port sits flush in the side of the fuselage. There you measure only the static pressure, the pressure of the air around you.