Is there any picture-in-picture mode in Mac OS? if yes, then how can I activate and use it?

Do you want to know if there is a picture in picture motion in Mac OS? This is possible as long as you would find the picture in the picture button. You can just open the video or image in iTunes. Next, you need to find the picture in the picture button. If you do not know how the button looks like, it is like two rectangles that are separated by one diagonal arrow.

This will make prompt the video or the image that you are looking at to go to the side of the screen so that you can play another video or look at another image instead. This can be very useful to use especially when you need to compare different media files easily.