What the Heck Is grain free forage replacer UK?

https://daily-horse-feed417.huicopper.com/equine-feed-for-electrolyte-support-a-useful-guide

Gentle horse feed for sensitive stomachs, designed to support comfortable digestion, hindgut health and better nutrient absorption. A reduced-starch fibre-rich formula with naturally sourced ingredients to help maintain calm digestion and overall health.