The easiest way to explain it is that the pitot tube is calibrated at sea level pressure and 15 degrees celsius. At these conditions IAS will equal TAS. As you climb the ambient pressure and temperature (standard day) will decrease. The pitot tube is not accurate at different pressure altitudes and temperatures. Remember the airspeed indicator measures ram air pressure. If the pressure of the ambient air decreases the pitot tube will not "capture" the ram air pressure correctly and thus not accurately display the airplane's speed. Best way to calculate TAS correctly is with an E6B. If you have a GPS on board it may do the calculation as well.
Also remember that regardless of the true speed through the air, aircraft limitations are still based on IAS.