Gold is traded against both the US dollar and Euro, and is traded in a similar way to any other currency pairs.
Buying the XAU/USD pair means the trader is going long gold and simultaneously selling the dollar. The price of the pair reflects the value of the metal in terms of the US dollar.
The XAU/USD pair represents the price of 1 troy ounce of gold. As the value of gold increases in relative to the US dollar, the price of the pair appreciates to reflect the increase in valuation of the metal and the decrease in the dollar.
On the contrary, if the price of gold declines, fewer US dollars will be necessary to purchase an ounce of gold, and therefore the price of the pair will decrease.