The problem facing municipal bond investors is a double whammy. First, an investor who buys a municipal bond at a premium over face value (which is common in this price-inflated environment) only gets back face value at maturity, and the difference reduces the total return of the investment.
Second, municipal bonds are callable by the issuer (unlike U.S. Treasury securities and many corporate bonds). When a bond is called, it is redeemed prior to the maturity date and the stream of interest payments is cut short. That reduces the yield that would be expected if the bonds were held to maturity. In muni bond lingo, the measure of this double whammy is called the “yield to worst.”