It is a hen that has been spayed by having her ovaries removed. This prevents the bird from using up energy by laying eggs.
The hen is then fed abundantly to fatten it. It may be kept in a coop for the final part of its life to restrict movement, and may even be force fed if necessary.
The birds are then slaughtered at around 11 or 12 weeks old.
Some chicken breeds are better used than others. A breed called la Flèche is used in Mans, France, which is famous for its Poulards
A Poulard is the female equivalent of a capon.
Poulard is also being used now as a term for leather made from chicken skin. Chicken skin dress shoes, anyone?