The pitot tube is a small but important part of your airplane's pitot-static system. It measures the pressure of the air entering its forward-facing opening as the airplane moves through the air. Combined with static pressure from the static ports, that information allows the airspeed indicator to calculate indicated airspeed.
So what happens when the pitot tube becomes blocked? Well, it depends on what's blocked.
If the pitot opening is obstructed but the drain hole remains open, the airspeed indicator will typically drop toward zero. if both the pitot opening and the drain hole are blocked, total pressure becomes trapped inside the line. Because the static port remains open, changing ambient pressure causes the airspeed indicator to behave like an altimeter: the indicated airspeed will increase during a climb and decrease during a descent.
Ice is one possible cause of a blockage, which is why many aircraft are equipped with pitot heat. But insects, dirt, and other debris can also obstruct the opening. A pitot cover helps keep unwanted visitors out while the airplane is parked.
Learn more about the pitot static system in this video excerpt from Sporty's Learn to Fly Course.
