A padlock icon, or lock icon, displayed in a web browser indicates a secure mode where communications between browser and web server are encrypted. This type of connection is designed to prevent anyone from reading or modifying the data you exchange with the website.
These websites are encrypted with the SSL / TLS protocol to secure transmitted data. Typically you would expect to see the padlock icon on websites where sensitive data may be transmitted, such as a banking website or online shopping site. In most browsers, users can click the padlock to learn additional information about the secure connection, including any permissions you have previously granted, information about the website and verification, the number of times you have visited the site, stored cookies, saved password and more.
Recommended Reading: Learn more in Webopedia’s HTTP and HTTPS definition.