What is an SSL certificate
In short
An SSL certificate encrypts the connection between your site and a visitor. Thanks to it the site opens on a secure https address and the browser shows a padlock.
Without SSL, data such as form input travels unencrypted. The browser warns that such a site is not secure.
An SSL certificate can be obtained for free today and is configured when the site launches.
Why SSL matters
Without SSL the browser shows a warning that instantly scares visitors away. Https is also a Google ranking factor. SSL is the mandatory minimum today.