A low contrast or “LowCon” filter is a technique used by some cameras and cinematographers to reduce the appearance of subject contrast and soften highlights. A low contrast filter is typically placed in front of a lens, but can also be an extra fixed layer in front of the camera sensor. It works by inducing veiling flare, usually similar to but much subtler than adding a thin layer of petroleum jelly to the front of a lens.
Low contrast filters have a variety of names, strengths and influences. More generally, the family of filters which soften contrast and detail are typically called low contrast, ultra contrast, fog, mist, softening, frost or diffusion filters. Some act more on the highlights or shadows, others act more uniformly on fine details — but all act to scatter light.
Consult with your specific filter manufacturer’s documentation for more on their particular implementation.
Learn more about LowCon filters in the RED 101 Article: Understanding Low Contrast Filters