Image: A kittle, folded up like a shirt.
A kittel is a garment you might see during the High Holy Days or at a wedding. It is a white garment made of lightweight fabric, usually cotton or a polyester blend. It looks a bit like a very light lab coat with a cloth belt, generally in a mid-calf length, and it is worn buttoned-up.
You may also see a kittel on the chatan [bridegroom] at a wedding.
In both cases, the kittel signifies the purity of heart one tries to bring to those situations. The color white is sometimes said to take its inspiration from this verse from the prophets:
Come now, and let us reason together, says the Eternal; though your sins be as scarlet, they shall be white as snow; though they be red like crimson, they shall be like wool.— Isaiah 1:18
A kittel may also be worn by the meyt [body of the deceased] in the coffin for burial. Usually it is paired with long trousers of the same material. The persons who wash the body for burial dress it before putting it in the casket. You are unlikely to see a meyt in a kittel because it is not the custom to view a body after death; Jewish funerals are always closed-casket.
This gives us another reason that some people choose to wear a kittel during High Holy Day observances. There is a focus on the end of life during those days, especially Yom Kippur. Wearing the garment in which one might be buried is a sharp reminder of our mortality.