This is called aqua regia.
"Aqua regia" is Latin for "royal water", because unlike most simple acids it can dissolve the "royal" metal gold. Aqua Regis also dissolved some other metals that nitric or hydrochloric acid alone cannot attack, but hold is best known.
In terms of chemistry aqua regia brings the best of two worlds. Gold is so noble that oxidizing power alone can't make it react, at least not in aqueous solution. But the oxidizing power of nitric acid plus the complexing power of chloride ion, from the hydrochloric acid, enables forming a soluble gold complex that makes the mixed acid, well, royal.