An "apostille" is a form of authentication issued to documents for use in countries that participate in the Hague Convention of 1961 (Convention Abolishing the Requirement of Legalization for Foreign Public Documents).
The apostille should be issued by the authority of the country where the document originates (i.e. Mexican documents to be used in another country, should be apostilled by the corresponding Mexican authority.
When an apostille is attached to a document, it is exempted from all form of authentication and no further validation is required.