Wikipedia has this explanation Chromatic Aberration:
Chromatic aberration is an effect resulting from dispersion in which there is a failure of a lens to focus all colors to the same convergence point. It occurs because lenses have different refractive indices for different wavelengths of light. The refractive index of transparent materials decreases with increasing wavelength in degrees unique to each.
Chromatic aberration manifests itself as "fringes" of color along boundaries that separate dark and bright parts of the image, because each color in the optical spectrum cannot be focused at a single common point. Since the focal length f of a lens is dependent on the refractive index n, different wavelengths of light will be focused on different positions.
All of that said, on plain english it just means sometimes you get some fringes of color around edges in your photographs.
Here is a video about Chromatic Aberration and how to deal with it in Lightroom:
Video by Anthony Morganti