A chromakey (color key) simply removes a solid color (usually green) from an image, to be replaced by a background image that's on a lower track.
If the image has color that's near to or the same as the key color, it will be keyed out. If the girl here wore green gloves, they too would be keyed.
Shot in front of a green screen, and placed on track 2... 
The Chromakey effect is applied, so the background image, on track 1, replaces the green... 
Removing complex elements from a concert still shot may require masking. Or perhaps sophisticated graphics software. For work with still images check out Photopad. For a more extensive explanation, look here.