an enclosure is a box where you can have the sub fixed. These can be custom made or factory installed. Companies like Rockford Fosgate, Infinity, Hertz provide enclosures. They have a more tight installation of the sub into the box and are better in terms of punchiness of the bass.
Custom made boxes are buying the sub seperately and getting a box made to fix the sub. They are good on output as well however not as good as factory enclosures JBL REF1240SE
Above is the link of a enclosure . The one which I got for my car.