Easy. Swipe your finger from the right edge of your screen toward left, it will pull out all the apps that work with multi screen. Then drag and drop the ones you want. You can resize as needed. Up to 4 apps at a time.
you can either drag and drop which limits you to 4 or you can have one open and then tap the apps and it creates free floating windows. In that case you will have 5....pretty crowded though
Sent from My Samsung Note Pro 12.2 SM-P905V using Tapatalk
My typical use was to surf the web full screen while having a video playing in a floating window . . . or surfing while chatting with Hangouts in a floating window etc.
While most stock Android ROMs include features that can only be considered bloatware, once and a while a manufacturer comes up with an innovative and useful feature that they include into their OEM products.