When you look at an image or a photo you’re looking at a refraction of light waves generated by the glass in a lens, and directed towards a camera’s film plane or image sensor. Sometimes that refraction distorts and generates something of a spectral deformity called a chromatic aberration. This happens when all the spectral colors aren’t focused to the same convergence point. The result is a shift in certain wavelengths from this point, which manifest as a hazed outline or fringe of a specific color at the border between a dark and bright part of the image.

Basically what that means is that different wavelengths, though representing the same points or edges of a photographed subject, are focusing at different positions.

There are two forms of chromatic aberration: axial and transverse. When different wavelengths are focused at differing distances from the lens, and thus at different focal planes, the resulting aberration is axial.  This looks like a border completely rimming the subject in whatever color wavelength is focused behind the film plane.

When wavelengths focus on the same focal plane, but end up having a different magnification point on that plane, thus creating two different color fringes on either side of the subject, it results in a transverse aberration.

Though you can see both forms of aberrations in a single image, you’re a bit more likely to just see a transverse aberration as axial aberrations are often corrected by simply stopping down your lens. Transverse aberrations are more a product of the glass in your lens. That being said, there are achromatic lenses which attempt to compensate for these distortions.