Chroma refers to the color content of a video image. In video work–as opposed to film–an image is broken down into a luminance component and two chroma sub-carrier components.
This is a legacy of the transition from black and white television to color. When color TV was introduced, rather than render existing TV sets obsolete [anvil on a TV set], broadcasters kept the original black and white broadcast, but added the chroma sub-components in addition. The result is that black and white TV sets only see the luminance signal and this display the black and white signal, but color TV sets are able to decode the chroma components of the signal as well, converting the result into an RGB color signal.
Digital video engineers have taken advantage of this separation of luminance and chrominance to significantly compress digital video.