SSL or Secure Socket Layer, is a security technology that encrypts data between your website and a visitor. It ensures that the transfer of sensitive data like passwords and credit cards are done securely. Using SSL on all pages is a modern standard, and Search Engines have made it a ranking signal in recent years.
SSL can often be switched on quite simply in systems like Wordpress, Wix etc. Often in more custom websites though, it may require the help of a technical resource to install and configure this on your website. After installation, test that your website loads successfully at a HTTPS:// location.