The higher the risk of a loan, the higher its interest rate. The higher interest rate compensates you for the greater chance that the borrower may default.
- Loans with a rating of A or B have lower interest rates and a lower risk.
- Loans with a rating of C have higher interest rates and a higher risk.
A higher interest rate does not mean you will earn more in the end. If a borrower defaults, you can lose interest and capital. So think about how much risk you can bear, and spread your money across many loans. See What does diversification mean?