A homograph is a word that is spelled the same as another one, but has a different meaning. Often, it is also pronounced differently.
One of the best examples of this is the pair of words spelled lead. If this is pronounced with a long E sound, it means to conduct or to be at the front of a group. When lead is pronounced with a short e sound, it means a type of metal that can be poisonous to people if they eat it.
For an example of homographs that are pronounced the same, you can think of bow. In one definition, it is a part of a ship or boat. In another definition, it is the instrument that, when paired with an arrow, can be used to hunt or fight.
Smile only has one meaning. It can be a verb or a noun, but both parts of speech refer to a movement of the corners of the mouth upwards. Because of this, smile is not a homograph.
