Legalization is a process of higher verification of documentation. This process is required for certain countries (those that did not sign Hague Convention). Depending on the type of document and country of intended use special legalization process must follow.

Multiple authorities must often be visited to attach their legalization stamps to prove that the document is valid and then super legalization stamp from the Embassy of country of intended use must be also enclosed.  (For example if you wish to use Criminal Record record legalization would require stamps by the Ministry of Interior, then Ministry of Foreign Affairs and finally super legalization by the Embassy of country of intended use.)