Rarity are how special the item are. There are 5 different type of Rarity which are Common, Rare, Epic, Legendary and Mythical.
Common are the one without any type of outline. Rare are the one with a bronze outline on it. Epic are the one with a silver outline. Legendary are the one with a golden outline and finally Mythical are the one with a red outline making it the most rarest item in the game.