Secure Login module enables the user login and other forms to be submitted securely via HTTPS, thus preventing passwords and other private user data from being transmitted in clear text. Secure Login module locks down not just the user/login page but also any page containing the user login block (or other forms that you configure to be secured).
For Drupal 7, Secure Login module enforces secure authenticated session cookies, thus preventing session hijacking. For previous versions of Drupal, PHP's session.cookie_secure flag must be enabled on the HTTPS site to enforce secure authenticated session cookies.
A word about Drupal 7's
Secure Login is intended for sites that want to offer anonymous sessions via HTTP or HTTPS and authenticated sessions only via HTTPS. Anonymous insecure sessions are migrated to authenticated secure sessions upon login, with all session data intact. Secure Login is designed to work with Drupal 7's
$conf['https'] setting at its default value,
If you were to change
TRUE, you would enable mixed-mode (HTTPS and HTTP) authenticated sessions: both secure and insecure session cookies are set when a user logs in to the HTTPS site. Other contributed modules, such as Secure Pages, may assist you with implementing mixed-mode authenticated sessions.
Pro tip: To help protect against SSL stripping attacks, you will need Strict-Transport-Security headers in your webserver configuration or HSTS module.