The page you linked to explains what it means. In this case, very narrow-bandpass filters were used when photographing the nebula; filters that pass such specific colours that it is possible to distinguish which elements had to be present to emit them. Three different photos were taken with three different sets of filters, and each of the images was treated as if it were a black-and-white (luminance-only) image. Two of those colours are very nearly indistinguishable by sight, and both are at values of red that would appear very dark to the human eye.

In order to make the final image, each of the luminance images was assigned a colour. Then the three images were combined into a single image.

Colour mapping is the process of assigning colours that way, creating a false-colour image that allows us to see things (details in composition, frequency distribution, heat, etc.) that we wouldn't be able to see otherwise. The original images may be infrared or ultraviolet, or even radio-frequency images, that have no "real" colour to the human eye. Sometimes the colour assignment is deliberately made in order to enhance the aesthetic appeal of the image, and the relative brightness and contrast of the component images is adjusted to create an artistic effect.