Boomerang uses the iPhone's camera to take bursts of ten photos at a time. After the burst is completed, Boomerang will stitch the ten photos together and create a 1-second looping video. The feature is a bit similar to Live Photos on iOS 9, but it's more closely related to looping .gif animations.
Fortunately, owning an iPhone 6S or 6S Plus is not necessary for Boomerang. The app supports all iPhones and iPod Touch devices running iOS 7.0 and up. Boomerang can be found for free in the iTunes App Store.