(excerpts from WP Engine's white paper, The Importance of SSL Security)

What is SSL?
SSL stands for Secure Sockets Layer. It’s a technology that encrypts all of the information sent to and from your site; meaning private data your visitors share with your site stays private.

How do I know if a site is secured by SSL?
It’s simple. If the web address starts with HTTPS as opposed to HTTP, then that site is protected by SSL (that “s” stands for security). There’s also a nifty little lock icon next to the URL that indicates a site is protected. If you use Chrome, the lock icon is green and looks like this (a grey lock indicates that a site has mixed content, meaning some pages are protected by SSL):

Why should I consider SSL? SSL ensures that hackers cannot see or intercept the data your users share on your site. Essentially, it creates a secure tunnel between the browser and the web server. It also verifies that your site really is who it claims to be, and it’s not an imposter. So, if you’re buying something from Amazon.com an SSL certificate verifies that you’re really sharing your data with Amazon.com, not some bogus site that’s going to steal your info.

You mentioned SEO... what’s SSL got to do with that?
Having SSL protection on your site also improves your site’s Google search rankings, as Google penalizes sites lacking an SSL certificate or with a certificate that is improperly set up. That means if your sites are protected by SSL, they rank higher than non-secure sites. There are some nuances, however, to get the most SEO bang for your SSL buck. It’s important to ensure all elements of your site use HTTPS.

That includes widgets, JavaScript, CSS, files, images, and your content delivery network (CDN). Also, you must use 301 redirects to point all of your existing HTTP URLs to HTTPS (which is done automatically if you secure URLs via the settings in the WP Engine user portal). If you don’t, you won’t get the SEO bonus that HTTPS promises. There is a very helpful piece on Moz.com that offers a ton of tips to help you get the most SEO bang for your SSL buck.