XAUUSD means the price of gold compared to the US dollar.
In trading, XAU = gold and USD = US dollar.
XAUUSD shows how much 1 ounce of gold costs in US dollars.
Many traders follow gold prices from global markets such as the London Bullion Market Association.
The price of XAUUSD changes because of:
world news
inflation
US dollar strength
In short, XAUUSD is used to trade gold against the US dollar in the forex market.