what are they? what can i use them for?

A re-roll is an item that allows you to "re-roll" the "dice" that gives your horse its particular shade of color or variation of markings. If your horse is chestnut, it could become flaxen chestnut or liver chestnut or get different markings. If it's tobiano it could get a different pattern. So it doesn't change the genetics at all, it just gives you a chance to get a slightly different picture on your horse page. Be careful if you have appaloosa because it could change which PATN-1 or PATN-2 pattern you get. It can also affect the rarity of your horse's color--and not always to make it rarer.