Historically, this has been reserved for sites that transmit sensitive financial or personally identifiable information such as SSN, DL, etc. In those cases (in the past), only those pages with forms were secured (HTTPS). The rest of the site was generally loaded over an unsecured connection (HTTP).
But recently, Google recommended that all sites use HTTPS on all pages, and Google's Chrome browser is starting to identify pages as "Non-Secure" which do you not use an SSL to secure the connection. It is also expected that
unsecured sites see an adverse impact on their Google search results.