Chroma-key is a feature that allows you to set a color/color range to be transparent for a background image or video to display through.

First thing you need to understand is how the tracks are arranged in the program. For Movie Edit Pro, the lower track number, the more behind the track. So track 2 is behind track 5. The reason this is important is that your background or the image/video you want to show through must be set behind the image/video you have for the chroma key effect.
 

Chroma key for the Magix software uses Green, Blue, Black, and White as defaults. I will try to demonstrate this using the following.
 

In the program, on track one I am going to place the background image and in track 2 I will place a foreground image (in this case it is a picture that I have altered to have green areas that I can use a chroma-key feature with).

Once I have both images in place, I select the foreground image or the one that has the chroma-key color (this object will highlight) and then I select the Effect tab from within the program. Then choose 'Video Effects' > 'Chroma key'. For this since my chroma key color on the edited photo is green, I select green, as you can see it allows the green area to become transparent and shows the background image.

That is how I use the chroma-key feature, other users might have a different way.