This is a document from an international treaty drafted by the Hague Conference on Private International Law. It is an international certification comparable to a notarization in domestic law given to someone interested to buy university degree.
Notarizations are also called notarial acts. It is the assurance by a duly appointed and impartial Notary Public that a document is authentic and the signature is genuine.
When you buy university degree, this document is also received. This legalizes documents issued in one country and intended for use in another country in order to be recognized as valid in the foreign country.