It reflects the additional return or yield an investor can earn from a security that bears more credit risk relative to one with lower credit risk. The credit spread of a security is usually measured by comparing it to a credit risk-free security, typically the government bond of the country of the issuer.
For example, a US corporate bond will be compared to a US treasury bond of the same maturity, and a Greek government bond will be compared to a German government bond with the same maturity. Both the US company and the Greek state will need to offer a higher return on their bonds because their credit is of lower quality than their risk-free reference. There is a specific credit spread for every maturity (each point along the yield curve maturity).