My name is Deepa and I work for the Social Media and Community Team at Dell. Although you have been provided with the correct answer but I would like to expand the same so that you will have a clear and better idea. It indicates that the page uses the SSL protocol (a data transfer security standard that encrypts data and authenticates the server and the integrity of the message) or the TLS protocol. This symbol indicates that all information, most notably banking details, is secured.
To be sure that these pages are indeed protected by SSL, you can also check the site's URL, which must begin https://, the “ s” indicating that this security system is in force. You can also click the padlock in the browser bar to view the identity of the Web site owner and also check that it comes from a valid Certificate Authority. This digital certificate is a document that an organization provides from its Web site to confirm their identity, and to enable a secure connection.
by happygeek: ADVERTISING LINKS DELETED - AGAIN!!!!!