When used in cooking, essence refers to an aromatic extract that is concentrated. Vanilla, truffle and almond are examples of essence that can also come in synthetic forms.
Essence is usually used to give something with less flavor a higher quality of flavor. For example, vanilla essence is sometimes used in plain frosting to give it a richer flavor and allow it to have a distinct taste. The richer flavor is not achieved through any sense of taste, but is used to help trigger olfactory senses in the nose. Olfactory senses will send signals to the brain that something is flavored similarly to the way it smells.
Essences of food are often used when making artificially flavored jellies, jams and drinks. The essences can drastically change the flavor, while the rest of the ingredients remain nearly the same throughout. Essences are an easy way to change the way that something tastes and smells without the worry of additional ingredients or major changes to recipes. Many essences are difficult to produce and may cost a lot of money. For this reason, most essences that are available for cooking are synthetic. Their chemical makeup is identical to that which would be found naturally, but they are cheaper to make.