This is a link to a wikipedia page with a nice table of bond ratings and historical default rates.
I was looking at some bonds rates BBB- and could not remember that default rate statistics. Based on this table, a corporate debt instrument with a rating of BBB- would mean there is a slightly less than 5% chance of default. Good to know so you can price your risk accordingly.
An example would be if you could get pick up a bond paying 5% at par at a discount of 10% to par. So instead a 5% yield it would 5.5% or so. Or a 10% bump in yield. With a 95% chance of being paid and getting your original money back.
Something to be aware of with debt instrument investing