A Notary Public is a specialist lawyer who can witness or certify documents for use in foreign countries. By virtue of the Notaries signature and official seal, authorities in other countries will be assured that the relevant checks have been carried out.
As an example, when buying a property abroad you may need to give your overseas lawyer power to deal with the purchase. A Notary Public can watch you sign a Power of Attorney and then notarise this document to ensure that it is accepted in the country involved.