Low contrast filters, as their name implies, reduce contrast levels. This allows more shadow detail to be visible. These glass filters are quite often used when shooting video to help give a film type look. White areas are retained and colours are muted.
Low contrast glass filters are available in densities 1/8, 1/4, 1/2, 1, 2, 3, 4, 5.
For more information, please see this Low contrast filters PDF
Please note that if you are ordering one of the sets of low contrast filters where the density choice is up to you, there will be space to specify your choices at the checkout.