SSL or Secure Sockets Layer is a security standard for websites that encrypts data sent to and from a website. This means that if you are on a compromised or hacked network, the data you are sending and retrieving from the website cannot be seen, only the web address you are accessing can be seen. SSL has since been replaced with a stronger encryption protocol called TLS or Transport Layer Security but getting a certificate is still often referred to as buying an SSL certificate.
Having a security certificate in place has a number of great benefits. As the site owner you are helping to protect your visitors and the Internet more generally from malicious actors. Google and other major search providers prefer secure website and so will rank your site higher than competitor sites, all other things being equal. Lastly, browsers such as Chrome, Safari, Firefox and Opera will often display warnings to users where a site does not have a security certificate in place.