An SSL certificate encrypts data that's transmitted from a website to a server. Data like passwords, credit card information, and other private messages are protected by that encryption.
In a nutshell, SSLs encrypt all data sent to and from a website, making it safer to transmit passwords, credit card numbers, and other personal information.
Websites with SSLs promote trust, both among your site visitors and with Google, which recognizes them as trusted and legitimate sites. These benefits can boost site conversion rates and search engine rankings.