A notary public is a person who can legally act as a witness during the signing of official documents. Notary publics provide verification of identity and prevent fraud. They do this by making sure that the person who is signing the document is actually who they say they are, and not another person signing the document in place of another, or forging a signature.
An example of a notary public could be an employee at a shipping company such as UPS who has been legally granted the power to act as a witness when people sign legal documents. So, for example, a person may come into the UPS office and have the notary public verify their identity while they sign a government document. The person can get the seal of approval from the notary public, which verifies their identity, and then they can ship the documents.