MacOS Sierra (10.2) is the latest iteration from Apple. It comes with a lot of new features, one of which is the picture in picture mode. This feature allows you to watch video while doing other tasks. The video player will be layered on the top of the screen while you can do any tasks behind it. You can resize the player as well as move the player to the location that is comfortable for you.
In this post, I will use an example of playing a Youtube video using this picture in picture mode. This is pretty simple to achieve. What you have to do is just look for a video to play, play the video, right click on the video, and choose the Enable Picture in Picture mode. Note that sometimes you might want to right click TWO (2) times in order for the correct context menu to show.
You can refer to this video on how this can be easily achieved.