Secure Socket Layer, which doesn't mean much to most of us. It started as a coding system to prevent hacking into financial websites and has sort of morphed into being a kind of seal of approval.
It is possibly a ranking factor for Google search, but more significant is that Google promote it, so sites without SSL are now shown on Chrome menu bars as "not secure", possibly enough to put some customers off!
The bad news is you have to pay for the "certificate" on top of your normal hosting. It also changes your domain style to "https" from "http", which can cause a few issues if you are migrating. Some of your code (canonical titles) will have to be altered and links into your website edited to the new version. Old links will still work, it's just that Google aren't really convinced until everything is "https"!
Not an urgent fix for a small informational or social type of site, but if you think that "not secure" tag is detrimental to business get it. Chrome are thinking of making it a red triangle!

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