A self signed SSL certificate is, as the name suggests, is one that is signed by itself.
Normally SSL certificates are signed by a certificate authority (CA). The CA goes through a validation process when a SSL certificate is applied for to make sure the applicant is who they claim to be. For example, if you were to purchase a SSL certificate from GoDaddy for your web site they would verify you own the domain before issuing the certificate to you. The certificate they issue will be signed by their root certificate.
A root certificate is one that is used to sign other certificates. The CA's root certificates are trusted by applications such a web browsers. Basically it lets the web browser know that the certificate was issued by the CA, and since the CA is trusted to validate certificate requests the certificate is trusted.
In case an invalid certificate is issued by a CA there are methods in place to revoke that certificate.