The real yield is the interest rate on US government bonds after subtracting inflation. It tells you how much you actually earn after prices rise. If the real yield is 1.87%, it means you earn 1.87% above inflation by lending money to the US government.
How does it affect gold?
When real yields are high, your money earns good returns in safe bonds. Why would you hold gold, which pays nothing? You wouldn't — so gold falls.
When real yields fall, bonds earn less. Gold becomes more attractive by comparison. So gold rises.
This is the single most important indicator for gold.