A certificate that encrypts the connection between a browser and your site, shown as a padlock and an https address.
SSL, known today by its modern technical name TLS, is a protocol that encrypts data exchanged between a customer's device and your server, so nobody in between can read or alter passwords or card details. A certificate tied to your domain is installed, and your address becomes https instead of http.
This is no longer optional: browsers warn about unencrypted sites, payment gateways require it and search engines take it into account. A site without it loses customer trust at first glance. Most modern hosting providers issue and renew certificates automatically. Practical tip: make sure every page and subdomain works over https and that http visits redirect automatically. If you manage the certificate yourself, set a reminder before it expires so your site does not suddenly break.