They have only one small stomach and their digestion is done in a different manner than the ruminants. This digestion calls for a higher quality and more specific type of forage. They need a forage that has a minimal size to it and won't do as well digesting a fiber that grossly exceeds this either. A good quality grass for grazing is the best case scenario. In winter, this translates into dried grass or hay.
Grain is the seed from grasses.
Horses rarely got a chance to eat much in the way of these seeds except when the grasses were mature enough to produce them. However, centuries ago, horsemen found that grains were a good source of concentrated and easy to store and transport feed. I am not a nutritionist so am not going to go into all the various forms of grains that are available to feed to your horses here. I would just like to give some practical ideas and background for a decent, down to earth feeding approach. This should always start with a basis of good quality grazing material, either grass or hay.