In order to access information you need to have a specific file in your disc, that will authenticate you. Those files are encrypted, to provice a high level of security. There are many standarts of certificates. The issuer is an entity that attests to the identity of the holder of the certificate. Certificates are very usefull when extra security is needed.
Certificates work in both ways.
The new browsers also have client-side certificates. So the server can know who YOU are.