Fun fact: that‘s how it was translated into german in the original release version of the sim. Pilotenrohr… pilot tube. Whatever logic tells what that should be 
The pitot tube is a tube usually attached to the wing or the nose of an aircraft that has a connection to the airspeed indicator. It measures the ram air, means the more the streaming air presses into the tube the more it pushes against a surface inside the airspeed indicator and the more speed the needle shows. If this pitot tube builds ice less air can press into it and the less air presses against the surface in the ASI. Pitot heat is an electrical heating of this tube to prevent and remove ice.
Normally you don‘t use pitot heat all the time but only in temps that allow for freezing conditions as hot air has another density than cold air and the indicated airspeed shows incorrectly (slower). The yellow notification is just that: a notification.