The free market is an idealistic liberal concept, which determines that exchanges of goods and services should not be regulated or intervened by the state.
Although such a concept is not 100% practicable in our material reality, those who apply it as a government policy usually result in a worsening of social inequalities and in prolonging or worsening economic and social crises.
Perhaps best laid down by Adam Smith, though already existing before his work The Wealth of Nations, free market policies encompass a large range of principles and laws that govern trade inside and outside a nation, so as to remove as many barriers as possible to trade.
Free trade was originally required by the bourgeoisie to destroy the old order of the nobility, who used to own monopolies and form cartels to reinforce their power. Today, free trade is also used as a vector for imperialism, notably in the Opium Wars that sought to dismantle China's protectionist trade policies in the late 19th century
Right-wing libertarians largely base their economy ideas of the ideal of a "free markets", and often think that modern neoliberal capitalism is not truly "capitalism", but rather "corporatism".