When running an app in a full-screen view and you receive a message, a floating popup will let you check the message without interrupting your current full-screen app. It allows you to run two apps side by side.
The screen will split into two halves giving space to both the apps equally. You can later resize the window by dragging the splitting dot.
To show you an instance, I played a video and received a WhatsApp message. Heres how it looks like, see the screenshot. Spilt Screen - Vivo Y55s