Green Screen effect is an advanced feature of Overlays. It makes a color range in the top layer of two overlay clips transparent, therefore the background video clip or image behind the top layer could show through. It's a special tool used during video making process so as to superimpose subjects onto an unlimited number of different virtual backgrounds, allowing you to make some really cool shots like creating a superhero flying through the sky. You can easily achieve this with the help of Filmora9 by using some green screen video footage clips or creating a pure green background when shooting your video footage.
Import both your background videos/images and green screen clips to the media library.
Drag the background clip to the video track on the timeline, and green screen to the another track as follows:
Move your cursor over to the green screen clip and double click to bring up the settings window at the left corner, and then just need to tick Chroma Key there. You will get the green screen effects automatically.
You also can refine video with tools like Tolerance, Offset and Alpha Channel to make background video clearer.
Once you are satisfied with the result, click OK at the lower right corner.