Data cited at: Numbeo
Crime Index is an estimation of overall level of crime in a given city or a country. We consider crime levels lower than 20 as very low, crime levels between 20 and 40 as being low, crime levels between 40 and 60 as being moderate, crime levels between 60 and 80 as being high and finally crime levels higher than 80 as being very high.
Safety index is, on the other way, quite opposite of crime index. If the city has a high safety index, it is considered very safe.