Legalisation is the act of making a document lawful.
In the UK, this is done by the Foreign and Commonwealth Office (FCO) and in many cases the Embassy of the foreign country in order for it to be considered genuine.
The Hague Convention abolished this between member states from 1965 for public documents. This procedure is known as the grant of an Apostille which is attached to the document by the FCO.
Please check with the country concerned whether legalisation is required.