Apostille is an official confirmation of the authenticity of your document (e.g., birth or marriage certificates, diplomas, powers of attorney, statements, etc.), as well as all the signatures on it. With an apostille, your document becomes valid beyond the country in which it was issued.
Previously, documents had to be legalized through embassies and consulates. However, this was a lengthy and expensive process. In 1961, the member states of the Hague Convention agreed to accept apostilled documents from each other without additional formalities.