Secure Socket Layers (SSL) is a technology used to encrypt the data that is sent between your server or host and a visitor to your web page. When SSL is activated, it becomes nearly impossible for an attacker to intercept data during transmission, making the submission of form, password or other encrypted data much more secure.
This feature gives you the option to enable SSL (if your server or host supports it) for all or part of your site. The options below allow you to automatically use SSL for large areas of your site, such as the login page, the admin panel, or the site as a whole. You can also enable SSL for any post or page by editing the content and selecting “Enable SSL” in the publishing options for that content.
Frontend SSL mode enables a secure SSL connection for the front end (public areas of your site). Disabling this disables front-end SSL checking. Enabling this places a checkbox on the edit page for all posts and pages (next to the publishing settings), so you can enable SSL for selected pages or posts and 'Entire site' will force the entire site to use SSL (not recommended unless you have a really good reason to use it)