SSL Certificates enable data encryption on the internet and allow data to be transmitted securely from a web server to a browser. With SSL, your website can use the https protocol and will display a padlock in end users web browsers to indicate the connection is secure.
Google wants to make the web safer and a big part of that involves making sure that the sites people access via Google are secure. That's why websites using SSL have been shown to benefit from higher ranking in search results.
A domain name is the web address or URL where visitors find your site. A custom domain gives your site a professional look, strengthens your brand, and helps visitors find you on the web.