The kittel is a simple white robe, usually of cotton or linen, worn over ordinary clothes. It is worn on the Days of Awe, by many at the Passover seder, by the prayer leader at the services for dew and for rain, and by a groom under the wedding canopy. The colour is read through the verse in Isaiah promising that sins red as scarlet shall be white as snow, and through the tradition that the ministering angels are clothed in white.
Its plainness is doctrinal rather than aesthetic. On a day of judgement the rich man and the poor man stand in the same unadorned garment, and no cloth, cut or ornament distinguishes them. In most versions it has no pockets, so that nothing can be carried in it, and no gold or metal, on the traditional ground that nothing recalling the golden calf should be worn on the day atonement is sought.
The same garment is used as a shroud. The burial clothes of traditional Jewish practice are white, plain and pocketless for exactly the same reasons, and many men are buried in the kittel they wore at their weddings and on every Yom Kippur of their adult lives. That is the point of it: the reminder of death is not added to the day of judgement as a decoration but is what the wearer is standing in.