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. Google has recently also made SSL a ranking signal, meaning an almost immediate boost to your site by implementing this.
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.