In short, it provides security for your website by encrypting all data transmitted over the internet to and from your website.
SSL stands for Secure Sockets Layer or its new version called Transport Layer Security (TLS) is an encryption protocol that secures the communications between your visitors and your website. SSL/TLS protects your visitors sensitive data against fraud and identity theft by providing secure encryption and by validating your website.
Google rewards websites that have SSL Certificates with better ranking on their results. Sites without SSL Certificates are flagged “NOT secure”, preventing visitors from easily accessing your website. (source)