A picture in picture (PIP) is a floating video viewer that makes it easy to watch videos and do other things. This feature will officially appear on Chrome 69. But right now you can test it.
This seems to be an official feature on Chrome 69. To try it out, you need to follow these steps:
Step 1: Download Chrome Developer 69 here and install. Don’t worry if you are using the current official Chrome 68, Chrome Developer 69 runs a separate process.
Step 2: Run Chrome 69 Developer, type chrome://flags/#enable-picture-in-picture, and change the status of this item to Enabled.
Step 3: Type in the chrome://flags/#enable-surfaces-for-videos address bar. Also Enabled this item up, then select RELAUNCH NOW to restart Chrome Developer and apply the settings you just made.
Step 4: The above two steps have enabled the picture in picture mode. Now go to YouTube, turn on a video, then right-click twice fast, select Picture-in-Picture to turn on the floating window for that video, very simple.