The Swanson Hut refers to a type of enclosed litter box that has a resemblance to the "Quonset Hut" that was utilized during World War 2. The idea was developed by a soldier named Swanson, hence the name "Swanson Hut".
The original Swanson Huts were steel drums which were cut in half, a doorway cut out, and places above a bed of sand. Cats that were kept as pets would enter these enclosures allowing for a reduction in fecal and urine odors - in the same manner as a litter box. The modern Swanson Hut is made of plastic or similar material and available at most pet stores.
References:
Swanson Hut - A little known fact
References
Swanson Hut Wikipedia(Text) CC BY-SA