Hen brooding is letting the hen take care of the newly hacked chicks, this is the natural method. It causes natural instincts of the chicks to be developed earlier as they can imitate what the hen does.
Nowadays, with many disease challenges that are present in farms, hen brooding is discouraged to avoid contracting disease from the hens. There are also limited number of chicks a single hen can take care properly which will result to lower efficiency and productivity, especially if you want to product a large or at a commercial scale.