What is Credit rating?
A credit rating is a grade from a rating agency that sums up how likely a borrower is to repay. On the scale used by S&P, BBB or better is called investment grade, while BB or lower is called high yield, sometimes nicknamed junk. A rating is an opinion and can be changed, raised or cut over the life of a bond.
Why Credit rating matters
Ratings give a quick sense of credit risk, which is a large part of why two bonds with the same maturity pay different yields.