Aqua regia is a concentrated mixture of nitric and hydrochloric acids. The two acids react to generate soluble nitrosyl chloride, water and nascent chlorine. Secondly, the gold is oxidised by nitric acid to gold(III) ions. This reaction is normally highly unfavourable and so very few gold(III) ions are formed if you put gold into only concentrated nitric acid. However, the nascent chlorine is highly reactive as it has an unpaired electron in a high energy orbital and it is able to react with the gold(III) ions to form the soluble tetrachloroaurate(III) complex. This removes the gold(III) ions from the equilibrium, driving the reaction to completion.