Chroma keying is a technique used to composite (layer) two images or video streams together based on color hues. Commonly referred to as green screen. Now, before you get excited about that fact that you can set up and implement the use of green screens on your own, adding a whole new dimension to your low-budget short film sci-fi vision, please hear our words of caution: Improper use of green screen will positively ruin your film. Use it sparingly – or get a professional involved – if this is a project you want to go places. But hey, if you’re just playing around, do just that. Play around. Have fun.

I found a helpful introductory video that explains how to get started with green screen. At about 7 minutes in it turns into a sales pitch, so go ahead and spend those last three minutes doing something else: