Its basically where you make a track around the edge, and or through your fields, the narrower confines encourage the horses to always keep moving to seek out food. This is far more natural to horses than a square field where they barely have to move to get at food which encourages obesity.
You can go as boring or as extreme as you like - if you google paddock paradise on google images you get wonderful miles of tracks, with natural rock crops/woody bits/rivers etc all included so its like a horsey playground. Mine is just a 12 ft track around 8 acres, water only in one place so they have to walk for it, 2 shaded areas and one section of trees they can get at
Theres a book and youtube vids plus a few thread son here i quite liked the sound of it when i came across it for the first time and if i ever have my own land i would love to implement a system. the basic concept is to keep the horses moving and give them different surfaces to walk across to help improve thier feet. can have grazing sections gravel sections hardcore sections woodchip sections etc etc