Yield to worst describes the worst possible annual return an investor might get on a bond assuming it is held as long as possible and it does not default. Normally that would be the definition of yield to maturity but some bonds come with the potential to be called (repaid early) at the option of the borrower. Yield to worst might assume that the borrower calls the bonds at the earliest opportunity though in many circumstances the yield to worst and yield to maturity will be the same.