What XAU/USD is
XAU is the ISO code for one troy ounce of gold and USD is the dollar. The pair is a price in dollars per ounce, which is why a move of one dollar is a move of one unit in the quote, not a percentage.
Two prices, two times
The London market fixes a price twice a day, and electronic markets quote continuously around the clock on weekdays. We use a continuous bank feed, so the number you see is a dealing price rather than a daily fixing.
What moves it
Real yields, the dollar, central bank buying and, more than anything, the cost of holding a metal that pays no interest. Gold tends to move on expectations about rates rather than on rates themselves.
What you are not buying
No metal is allocated, stored or delivered, and you are not entitled to any. You are taking a view on a published number for a fixed period, and that is the whole product.