Governments have a supreme law called Constitution, from which the rest of the legal system in the country will be derived. It is issued from a Consitutional power and needs agreeement from the inhabitants in this country to be approved (by voting at referendum for example).
A constitution is a document of a legal and political character, which establishes the separation of powers that rule this country (three in total: parliament, goverment and courts) and guarantees the rights and states the obligations of its citizens.