Legalization is a process by which the authenticity of a document is confirmed for use abroad. It involves a competent authority certifying that the signature and/or stamp on the document is genuine. Legalization says nothing about the content of the document, but guarantees that it is officially recognized in the destination country.
For countries party to the Apostille Convention, such as the Netherlands and Morocco, an apostille is often sufficient. This is a simplified form of legalization.