An ALL ALL robe is a piece of visible armor with a robe's appearance that can be worn by characters of any race or class, even ones who cannot usually wear robes. The Shade Silk Robe is an easy-to-craft example, but there are several others.
Some races do not have a robe graphic programmed into the game for use with their character models. Such races will not show a robe graphic even if an ALL ALL robe is equipped. The only races this happens to are those who cannot be any of the traditional robe-wearing classes (monk, necromancer, wizard, magician, or enchanter). Thus, the list of races who cannot display a robe graphic includes barbarians, dwarves, half elves, halflings, ogres, trolls, wood elves, and Vah Shir.