Yield to maturity or YTM for short is the internal rate of return a holder of a bond would receive if a bond is purchased at the market price. It is some time also known as the redemption yield and is commonly used to evaluate bonds.
Yield to maturity is considered to be one of the most accurate internal rate of return measures. It still however suffers from many of the flaws any IRR measures suffer from. YTM is considered to be more accurate because of the qualities that bonds posses make these flaws less severe. Qualities such as positive cash flows and the rate at which the incomes from the bond can be invested.
To fully evaluate a bond however you also need to consider if the risk is compensated by the yield spread and how that bond compare to similar bond on the market.