Coupon yield is like getting a piece of candy for owning a special paper called a bond. When you buy a bond, you give your money to the company or government that issued it. They promise to give you back your money plus a little extra (which is like getting a piece of candy). The extra money they promise to give you is called the coupon yield.
This extra money is usually given out once or twice a year and is a set amount. So, if you own a bond that promises to give you $5 every six months as the coupon yield and you bought the bond for $100, that means you will get $5 every six months until the bond matures. When the bond matures, the company or government will give you back your $100.
So basically, when you own a bond you get a little extra money as a reward, like getting a piece of candy for being a good kid.