What the Heck Is slow release fibre feed?

http://www.albanynewsnow.com/news/story/540487/with-half-of-britains-horses-overweight-bifid-launches-grainfree-lowsugar-feed-for-the-nations-gooddoers.html

Gentle horse feed for sensitive stomachs, designed to support comfortable digestion, intestinal health and easy nutrient absorption