Imagine all possible colors as living within a 3D coordinate system. A colorspace is the particular system being used in this mapping. The neat thing about color spaces is that information that is difficult to get at in one color space can be nice and easily acquired in a single channel in another color space. A good example is the Lab space which neatly places all luminance in the L channel, leaving the color information in the a and b channel. This means that the color values of the image can be edited without affecting its luminance (and visa versa). This is slightly advanced territory and is not something that you are ever going to want to do often, however when you need it you are grateful for the fact that its there.