This guide explains in detail how to use Chroma key node.
In short, chroma keying (or greenscreen removal) is a process of taking 2 images and compositing them together. By far the most common usage is to replace a background of an object with something else.
The above operation can be reproduced with the following layout:
The Input represents the image with a green screen and Background represents the background image. The color input should signify the color of the green screen whereas Mask range represents the range of the color that should still be considered as the green screen. If Mask range is too big, parts of the foreground will start to blend into the Background image and if it’s too small, you will start to see a bit of greenscreen.
If you check the Output mask you’ll see the mask that is used to remove the green screen from the foreground.