Yeah, it's an app already installed in your phone (unless you're on a carrier version that disabled it, but I don't think any are doing that) that basically allows you to have password-protected files, documents, and apps, and it also allows for 'double' app installation, so you can use the same app with two different accounts and run at the same time (like what Parallel Space does).
The Note 8, on top of things, now supports 'two accounts' on some apps (Facebook, Messenger, Whatsapp, etc.) without even using Secure Folder; you just activate the feature for the app and done, you can use two accounts. Not all apps support this 'native' feature, though, so for those you would have to resort to a second installation in Secure Folder (or apps like Parallel Space if you're so inclined).