The word mole has a number of definitions, but the five most common uses of mole deal with dermatology, zoology, espionage, the culinary arts, and chemistry.
First, a mole is a congenital (meaning you're born with it) spot on a person's skin. Sometimes they're dark, or raised, or hairy. And sometimes they're all three.
Second, a mole is a (usually) furry little animal that spends most of its life underground, burrowing through the dirt and eating insects.
Third, a mole is someone who infiltrates and gains acceptance within an enemy agency or government. In the world of espionage (or at least in espionage novels), this type of mole is also known as a double agent.
Fourth, mole (pronounced "moh-lay") is a spicy Mexican sauce that is used to season meat and poultry. Though recipes for mole are as varied in Mexico as recipes for barbecue sauce are in the United States, most mole recipes (especially the good ones) include bitter chocolate in their ingredients.
Lastly, a mole is a measurement of chemical weight. One mole of a chemical has a weight in grams equal to the total of the atomic masses of its constituent elements. For example, carbon has an atomic mass of 12.01, and oxygen has an atomic mass of 16.00. One molecule of carbon dioxide, which contains one atom of carbon and two of oxygen, has a total atomic mass of 12.01 + 16.00 + 16.00 = 44.01. One mole of carbon dioxide, then, is 44.01 grams.