In optics, chromatic aberration is caused by a lens having a different refractive index for different wavelengths of light (the dispersion of the lens).
In real ife what it means is that in some lighting situations my ultra-wide angle lens shows some abnormalities in the form of colored fringing on objects to the far left and the far right of my images as evidenced in my recent photo of the lightning strike over 610 in Uptown.
This is taken from the left side of the original image:
As you can see, the building has a red border along the right edge that is also noticeable along the top of the tree line. If you look at the HDR image I rendered from the original RAW file you will see that the cromatic aberration is much more pronounced.
This is pushing me to learn more about post processing software tools. I have been experimenting the Adobe Camera Raw plugin for Photoshop and the initial results are promising: