A notario público in Mexico is a lawyer appointed by the state and vested with public faith (fe pública). Nothing like a US notary who witnesses signatures: the notario drafts and authenticates the escritura pública, verifies the chain of title, calculates and withholds the transaction taxes, and registers the property.
In practice
In a land purchase the notaría is the operational center of the closing. It pulls the folio real and the lien certificates, confirms who holds title and how they got it, checks that property taxes are current, withholds the acquisition tax and — for a foreign buyer — creates the fideicomiso inside the same instrument. It also orders the appraisal (avalúo) used as the tax base and files the transaction's tax returns. Notario fees typically run 0.5% to 1% of the price, inside the 6% to 8% closing costs that are normal in the Riviera Maya — ask for the itemized estimate in writing before you schedule the signing.
Two things worth knowing before you sit down to sign. First, the buyer can usually choose their own notaría, even when the seller proposes one. Second, and more important: the notario is neutral by design. They attest to what happens and safeguard the legality of the act, but they do not negotiate for you and they do not represent you. The person defending your interests at that table is your own real-estate attorney.
Watch out for
A seller who insists on "their" notario and assures you no other advice is needed. Also the assumption that a Mexican notario is the equivalent of a US notary public — the powers and the liability are not comparable. And signing without having read the draft deed: you can ask for it days in advance, and you should.
Related: Escritura pública · Fideicomiso · Zona restringida · Guide: lawyer or notario?