A notary public is an individual who is authorized by a specific jurisdiction (state, county, parish, etc.) to perform specific duties, but the notarial act that is most relevant to real estate deeds is acknowledging signatures. Acknowledgements are important because they certify that the notary verified the grantor's identity and witnessed him/her execute (sign) the deed.
State laws vary on whether or not unrecorded deeds must be notarized in order to be valid between the parties, but they unanimously agree that all instruments submitted for recording must contain a notary's acknowledgment. By affixing their certificate to real estate deeds, notaries act as official witnesses and render the instruments admissible as evidence. This requirement is intended to preserve the integrity of public records and to protect the rights of property owners.