An SSL Digital Certificate is a digital representation of information that a Web site purchases from a trusted third party Certificate Authority, which verifies and communicates the identity of the site, validity period of the site's certificate, the site's public key (for encryption and decryption purposes), and the Certificate Authorities identity.
An SSL Digital Certificate allows someone browsing to a site to know that they are directly talking with that site and that all information being passed between that site and their computer is encrypted and safe from anyone else seeing it.
NOTE: Although a valid SSL Digital Certificate verifies company information and data integrity, it does not mean that a company is "good" or that it will deliver goods and/or services as expected. As always, this is an area that the consumer must research and assume responsibility for.