High Yield Debt is a nice way of saying "junk" debt, i.e., debt that is considered speculative grade. As you would expect, the yield on such debt, due to higher default risk, is higher than standard investment grade debt. However, with historically low interest rates, even the yields on "high yield" debt don't look so high any more. See article here, International Financing Review.
Dolvin, Steven D., "High Yield Debt" (2012). All Chapters. 41.