The yield-to-call is the yield of a bond assuming that the bondholder were to buy and hold the security until the call date. The calculation is based on the coupon rate, length of time, and market price, and is only valid for bonds called prior to their maturity.
For example, suppose that a municipality issues a callable 10-year $1,000 par value general obligation bond with a call price of $1,500 that can be exercised after five years. The yield-to-call would provide an investor with the return they would receive if it were called after five years.