How to place an overlay video with a transparent background upon another video/image? First off, I'm a novice to node compositing, so I might not understand some jargons. 

Below is the original video.  

Next photo: the white background is removed (my first time with invert node). The background is turned transparent. Now how do I lay this overlay upon another video or image with any different colour? 
I'd like to try medium brown ground or yellow for the lower ground.
The next day... 
I have tried Alpha Over as was suggested. Initially, it was tricky and difficult to work with nodes, while making mistakes. Now, I have finally achieved the desired result and here I enclose the screenshot.