Comets that pass near the sun regularly loose mass (by way of their tails), and so they only have a relatively short life-time. Short period comets would disintigrate and disappear in a time that is far less than the four billion years since the formation of the solar system (and presumably the short-period comets). The creationists want to use the fact that there are short period comets as proof the solar system cannot be billions of years old.
The Kuiper disk and the Oort Cloud were hypothesized to explain the existence of these comets. The creationists want to say that this is all made up in a desperate attempt to save "evolution". Well, it appears that the existence of these structures are being confirmed.