This is the score that grant independent companies on the creditworthiness of a company or country. These companies use a scale with letters, where AAA is the best and D is the worst. The rating agencies more important are Moody´s, Standard & Poor's and Fitch. 

S&P and Fitch use the same terms. However Moody´s is different. For example the worst in Moody´s is C and the worst in the others is D.

On the other hand the three do a different between Long-term and Short term.

The rating description from best to worst are prime, high grade, upper medium grade, lower medium grade, non-investment grade speculative, highly speculative, substantial risk, extremely speculative, default imminent and in default.

Nowadays, the best countries in the rank are Norway, Luxembourg, Singapore, Sweden and Switzerland.