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Halo refers to light and/or dark seams along strong contrast edges often caused by certain image manipulations. Most operations that modify local contrast tend to produce halos, such as f.e. in photoshop Shadows/Highlights or Unsharp Mask with a high radius as it is often used for contrast enhancement. Some HDR compression tools show haloing, too.

Haloing can be avoided by either not using this tools or use other ones with edge preserving blurring algorithms which do not produce halos, like the Contrast Masking Actions do. Here an example with darkened sky. One with Shadow/Highlight and Unsharp Mask with a too low radius of 30 pixels and one with Contrast Masking Actions and Unsharp Mask with a higher radius of 70 pixels.

Unmodified image
Heavy halos due to wrong use of Shadow/Highlight and Unsharp Mask
No halos with Contrast Masking Actions and higher radius Unsharp Mask