A callable-bond gives the issuer an option to repay early, usually when rates have fallen and it can refinance cheaper. Yield to call reruns the yield-to-maturity maths with the call date as the end date and the call price as the redemption amount.
Because the issuer exercises when it suits them and not you, the call caps how high a premium bond's price can go. Buyers of high-coupon bonds trading well above par should assume the call happens.
Example: a 7% bond trades at 108, matures in eight years, and is callable in two years at 102. Yield to maturity is about 5.7%, but yield to call is only about 3.9%. The call scenario is the one to price off.
Related: callable-bond, yield-to-worst, yield-to-maturity, premium-bond, negative-convexity