Weighted Average Credit Rating
  
See: WACC.
You're a bond portfolio manager. You have in your portfolio a bunch of A-rated U.S. Government paper; you also have a bunch of C-rated crap. Junk. The credit ratings of the bonds in your portfolio are all over the map, er, credit spectrum. If you have 80% of the portfolio in junk with very low ratings (but very high yield), then your weighted credit rating will be low. Like the amount of sleep you get at night thinking about all the risk you're taking.