SSL / HTTPS
SSL is the protocol that encrypts the exchange between a browser and a server. HTTPS is the secure version of HTTP that uses it.
Without HTTPS, data exchanged between a visitor and a site (passwords, forms, payments) travels unencrypted and can be intercepted. The SSL certificate encrypts that connection and shows the padlock in the browser.
Google treats HTTPS as a ranking signal and flags plain HTTP sites with a warning. For a site meant to convert visitors into clients, a missing padlock breaks trust before the page is even read.