It's a part of how planes measure their air speed. When you stick your hand out the window in your car you can feel the air pushing it back. The drag of the air trying to get around your hand creates a high pressure area in the front. How high that pressure is depends on how fast you're going. This is the ram air pressure (from the air getting rammed into your hand by your speed through it).

It also depends on the ambient pressure, which comes from altitude and temperature and humidity.

The pitot tube sticks out from the front of the aircraft, typically somewhere on the nose, pointing into the oncoming air. The hole at the front of the tube allows a very predictable amount of air to be rammed into it, so you get a very predictable amount of ram air pressure per unit of speed through the air.

A static port is like the pitot tube but is pointed down or back, so that there's no ram air pressure. The pressure in it is only affected by altitude, temperature, and humidity. The affect of altitude is very predictable.

The pilot gets information from weather stations and ATC about local conditions to adjust for temperature and humidity. For the static port, that just leaves altitude, which is how the plane knows its altitude. Then, the instruments can compare the ram air pressure from the pitot tube and "subtract" the ambient altitude pressure to derive the pressure that comes only from airspeed. Knowing that, you can know the airspeed.

TL;DR: the pitot tube sticks out from the nose usually and uses the air being rammed into it to determine the airspeed, with some additional input from another port which measures ambient pressure.